During the course of screening for new antimicrobial antibiotics, the cultured broth of microorganism No. 1117 (identified as Streptomyces avidinii, isolated from a soil sample collected in Ikeda, Hokkaido, Japan) showed marked antimicrobial activity against Gram-positive bacteria and was found to contain a new macrolide antibiotic named aldgamycin G (1) together with aldgamycin F (2). This communication reports the isolation and characterization of aldgamycin G and F. The organism was cultured at 27°C for 48 hours in a 300-liter fermentation tank. The culture filtrate was processed via Diaion HP-20 column chromatography, followed by silica gel column chromatography and Toyoparl HW 40F column chromatography, yielding pure aldgamycin G (1, 77 mg) and aldgamycin F (2, 43 mg). Physico-chemical properties of 1 and 2 are shown in Table 1. The 13C NMR spectra (Table 2), mass spectrometry, and COSY/NOESY spectra indicated that aldgamycin G (1) is an 8-dehydroxy derivative of aldgamycin F (2). Antimicrobial activity (Table 3) showed that aldgamycin G (1) exhibited higher activity against Gram-positive bacteria than aldgamycin F (2), with MICs comparable to or better than josamycin against strains like Staphylococcus aureus Smith and Bacillus cereus IAM1729. In vivo chemotherapeutic activity (Table 4) revealed that aldgamycin G (1) had an ED50 of 0.6 mg/mouse against S. aureus Smith-infected mice, superior to josamycin (1.2 mg/mouse) and erythromycin (1.0 mg/mouse), though it was less effective against some clinically isolated strains such as Streptococcus pyogenes DP type 2.