RECENTLY, we reported the isolation and structural elucidation of maytansine (l), a novel antileukaemic ansa macrolide tumour inhibitor from Maytenus ovatus. We now describe two new and potent antileukaemic ansa macrolides isolated from Maytenus buchananii (Loes.) R. Wilczek, maytanprine (2) and maytanbutine (3). The new principles showed antileukaemic activity against P-388 lymphocytic leukaemia over a 50-100-fold dosage range at the pg kg-1 level. The alcoholic extract of stems collected in Kenya in 1970 was fractionated by the procedure summarized earlier, to yield a highly enriched concentrate. Preparative t .I.c. on silica gel afforded maytansine (1) (0.00015%) and two bands of higher R,. Purification by high pressure liquid chromatography yielded maytanprine (2) (0.00012%), C,,H,,ClN,O,,, m.p. 169-170", [or]? - 125" (G 0.0559, CHCl,), and maytanbutine (3) (0.00009%), C,,H,,CIN,Ol,, m.p. 170--171", [a]? - 122' (c 0.0492, CHC1,). The i.r. and U.V. spectra of (2) and (3) were almost identical to those of (1). The principal peaks in the mass spectra of (1)-(3) are listed in the Table; the compositions of the fragment ions were determined by high resolution m.s. The m.s. characteristics indicated that (2) and (3) have ansa macrolide structures similar to (1) except for differences in the Rgroups of the ester side chains. The relationships among the three compounds were confirmed by comparisons of their n.m.r. spectra. Thus, the n.m.r. spectra of (2) and (3) differed from that of (1) solely in the signals corresponding to the terminal R-CO groups. The n.m.r. signals for the propionyl group of (2) [T 8.82 (3H, t, J 7), 7.63 (lH, m), 7-59 (lH, m)] indicated the non-equivalence of the methylene protons, and this was confirmed by spin-decoupling studies. The n.m.r. signals for the isobutyryl group of (3) [T 8.88 (3H, d, J 7), 8-81 (3H, d, J 7), 7.20 (lH, m)] suggested the non-equivalence of the two methyl groups, and this also was supported by the results of spin decoupling and solvent shift studies. The signals for the C-2' N-CH, of (3) [T 7.13 (3/4H, s), 7.08 (2-1/4HJ s)] indicated that the rate of rotation about the carbonyl to nitrogen bond was reduced by the steric interaction of the isopropyl group and the aromatic ring. Studies of other active principles of M. buchananii are in progress.