A new antibiotic, trienomycin A, has been isolated from the culture broth of Streptomyces sp. No. 83-16. The physico-chemical characteristics of the antibiotic suggested that it belongs to the group of ansamycin antibiotics with a triene moiety in the molecule. The molecular formula of trienomycin A is C36H50N2O7 (MW 622). The antibiotic possesses potent cytocidal activity against HeLa S3 and PLC hepatoma cells at concentrations of 0.1 and 0.01 micrograms/ml (IC50 value) respectively. However, trienomycin A showed no antimicrobial activity against the bacteria, fungi and yeasts examined with the exception of weak activity versus Piricularia oryzae at a concentration of 1,000 micrograms/ml.