A new antibiotic, named 'desertomycin' after the African desert sand source of Streptomyces flavofungini (a previously reported strain producing flavofungin), was found in the fermentation fluid and crude flavofungin of this strain. The original strain produces more of this antibiotic than flavofungin, while its natural variant synthesizes more flavofungin. Desertomycin was extracted from the fermentation broth and mycelium with organic solvents and obtained as hexagonal crystals. It is a uniform snow-white, glittering crystalline product with a tentative empirical formula C₃₃H₆₀₋₆₂O₁₄N, melting at 189-190°C. It has low solubility in water, absolute alcohols, ether, and acetone, but higher solubility in alcohols containing water, and its neutral aqueous solution is very stable. Desertomycin contains no sulphur, halogen, N-methyl or acetyl groups but has a C methyl group; it forms acetylated and hydrogenated derivatives, decolorizes permanganate or bromine solutions, and gives positive ninhydrin and Kuhn-Roth C-methyl tests. Crystalline desertomycin showed antimicrobial activity against various bacteria with minimum inhibitory concentrations (MIC) ranging from 5 to 25 µgm./ml. The LD₅₀ values in mice were 1.35 mgm./kgm. (intravenous), 2.6 mgm./kgm. (intraperitoneal), 5.3 mgm./kgm. (subcutaneous), and 12-15 mgm./kgm. (oral). It exhibited significant cytotoxic effects: in vitro, it inhibited the life-activity of leukæmic cells at 0.7 µgm. and Ehrlich ascites cells at 10 µgm. It had a cytostatic effect (10 µgm.) and, at higher concentrations (50-100 µgm.), a cytolytic effect on fibroblast, HeLa, and Crocker cells in tissue culture.