WS-30581 A and B are antiplatelet agents produced by a strain of Streptoverticillium waksmanii. The compounds were purified by solvent extraction and column chromatography, and identified as new pimprinine analogs on the basis of their physico-chemical properties. The producing organism produced pimprinine along with these compounds. Pimprinine and the two new analogs have potent inhibitory effects on platelet aggregation.