Chetomin is a metabolite of Chaetomium cochliodes and C. globosum. It is produced by C. cochliodes on a denned medium and the yield is increased about 50-fold by the addition of corn steep liquor to the culture medium. Five single spore isolates of C. cochliodes (HLX 440) all produced chetomin and one of them produced appreciable quantities of a metabolite related to chetomin but having a S:N ratio of 1:1. Previous reports of the antibacterial activity of chetomin were confirmed and the antibiotic was shown, in addition, to inhibit the mycelial growth of some fungi. At 0.02g/ml, it inhibited protein synthesis in cultures of HeLa cells. Its oral LD50 in rats was 75mg/kg, and in turkeys, 30mg/kg. No pathological changes were observed in lambs dosed orally at 30mg/kg but a single intraperitoneal dose of chetomin at 5mg/kg resulted in the death of half the animals treated. Sublethal oral doses in all the animal species mentioned resulted in loss of body weight.