Lokysterolamine A and B are two steroidal alkaloids isolated from an undescribed species of the sponge genus Corticium, collected in Sulawesi, Indonesia. Compound A is N,N-dimethyl- and B is N-acetyl-4β-hydroxy-3-epi-plakinamine A. The crude extract of a sponge of the genus Corticium, collected in Indonesia in October 1992, showed antimicrobial activity against Bacillus subtilis. As a result of bioassay-guided fractionation two new steroidal alkaloids, 1 and 2, were isolated. The compounds bear a skeletal relationship to the previously described plakinamine A [3]. In order to indicate their chemical nature, they have been called lokysterolamines. The hreims of the major metabolite 1, together with its 13C-nmr spectrum (Table 1), indicated a molecular formula of C31H51N2O. Its 1H-nmr spectrum (Table 2) showed two methyl singlets at δ 1.04 and 0.60, and a methyl doublet at δ 0.88, which were correlated by HMQC to carbons at 15.8, 12.5, and 19.5 ppm. This suggested a steroidal character of 1 with a deshielded C-19 methyl group. An ir absorption at 3400 cm-1 and a carbon signal at 70.4 ppm correlated to a 3.92 ppm 1H triplet suggested the presence of an OH group in 1, which was subsequently located at C-4 by HMBC. Its axial conformation was compatible with the chemical shift of H-19 (δ 1.04). A small H3,4 coupling constant, J~2.5 Hz, and an nOe experiment, which showed correlations of H-4 to NMe2, H-3, and H-5 confirmed the stereochemistry (Figure 1). When 1 was dissolved in CD3OD acidified with TFA, 1H-nmr analysis of the resulting salt confirmed that 1 bears a dimethylamino substituent. The signals of the 6H singlet and H-3 multiplet, 2.34 and 2.01 ppm, were shifted downfield by 0.56 and 1.07 ppm. The lowfield shift of C-3 (69.2 ppm) suggested β-orientation of the dimethylamino group; while 3α aminosteroids are rare, the corresponding 13C-nmr chemical shift is observed at much higher field (2). This observation also was in agreement with the value of the diaxial H2,3 coupling constant of the protonated form, J=12.5 Hz, and finally was confirmed by nOe experiments, which showed a mutual correlation of H-3, but not of Me2N, to axial H-5. The configuration of the C-3 substituent in 1 is epimeric to that reported for plakinamine A 3 (1), which was established by comparison of 13C-nmr shifts with synthetic 3α-amino-5α-ergosta-7,22-diene. In addition, the 13C-nmr spectrum of 1 revealed five low-field signals. Two of them, at 119.4 and 139.9 ppm, were assigned by HMBC to a Δ7 double bond in close analogy with plakinamine A [3]. Signals at 176.2, 137.4, and 133.3 ppm, and the uv absorption at 247 nm (ε 8200) were in good agreement with those reported for 3. Comparison of the 13C-nmr data (Table 1) established the nature of the side-chain and the stereochemistry at C-20. The small differences in the chemical shifts of C-23 and C-25 in 1 and 3 may be due to different sample preparation, since we noticed δ-values are somewhat concentration dependent. However, our HMQC and HMBC correlation data suggest that the C-22 and C-28 chemical shift values in plakinamine A should be interchanged. NOe correlations of H-26 to H-22 and H-27 to H-28 allowed chemical shifts assignments for the methyl groups (Figure 1). Hence lokysterolamine A [1] is N,N-dimethyl-4β-hydroxy-3-epi-plakinamine A. The hreims of the minor metabolite 2 together with its 13C-nmr spectrum indicated a molecular formula of C31H48N2O2. Comparison of the spectral data with those of 1 (Tables 1 and 2), the presence of a 3H singlet at 1.94 ppm, and a low-field carbon signal at 172.3 ppm, together with a strong ir absorption at 1660 cm-1, suggested an acetamido moiety at C-3 instead of dimethylamino. The 1H-nmr signals of H-3, H-4, and H-29 in MeOH-d4 overlapped. A spectrum recorded in C6H6-d6/CDCl3 separated them and revealed coupling constants that allowed assignment of stereochemistry in ring A. An H-4 triplet, J=2.5 Hz, indicated an axial OH group, and the H2,3 diaxial coupling constant of H-3, J=12.1 Hz, suggested equatorial conformation of the acetamide. This was supported by nOe experiments which showed the same correlations as described for 1. In addition, the spectrum in C6H6-d6/CDCl3 showed an NH doublet at 5.48 ppm, coupled to H-3 (J=8.3 Hz). Plakinamine A [3] was reported to possess antimicrobial activity against Staphylococcus aureus and Candida albicans (1). Lokysterolamine A [1] had in vitro activity in mouse lymphoid neoplasm (P-388), human lung carcinoma (A-549), human colon adenocarcinoma (HT-29), and human melanoma (MEL-28) assays. In addition, it showed medium immunomodulatory activity (LcV/MLR>187), and antimicrobial and antifungal activity against B. subtilis and C. albicans (Table 3).