Triacsins A and B, new inhibitors of acyl-CoA synthetase, were isolated from the cultured broth of Streptomyces sp. strain SK-1894. The structurally related compounds WS-1228 A and B, known as hypotensive vasodilators, were also found to inhibit acyl-CoA synthetase. The four compounds have in common a N-hydroxytriazene moiety in their structures. The IC50 values for triacsin A and WS-1228 A were 5.5 and 3.6 micrograms/ml, respectively.