A new platelet aggregation inhibitor, aggreceride, was isolated from the culture broth of Streptomyces strain OM-3209 during screening for such inhibitors from actinomycetes. The work describes the screening, fermentation, isolation, structure elucidation, and biological properties of aggreceride. Screening used washed rabbit platelets with thrombin or ADP as aggregating agents, measured by visual inspection. Fermentation was conducted in a 50-liter fermentor (30 liters of medium containing glucose 0.1%, starch 2.4%, peptone 0.3%, meat extract 0.3%, yeast extract 0.5%, agar 0.1%, CoCl2·6H2O 20 mg/liter, CaCO3 0.4%, pH 7.0 prior to sterilization) at 27°C with agitation (250 rpm) and aeration (35 liters/minute), reaching maximum production at ~70 hours. Isolation involved ethyl acetate extraction, silica gel column chromatography (developer: chloroform-methanol, 10:1 to 5:1), rechromatography (benzene-acetone, 7:1 to 2:1), and preparative thin-layer chromatography (benzene-acetone, 3:1), yielding 27 mg of aggreceride as an amorphous white powder. Aggreceride is a 1-monoglyceride consisting of major component A (C18H36O4, [α] -9.7°(c 0.33, CHCl3)) and minor components B (C19H38O4) and C (C20H40O4); alkaline hydrolysis afforded glycerol and fatty acids (12-methyltetradecanoic acid for A, 14-methylpentadecanoic acid for B, 15-methylhexadecanoic acid for C) confirmed by GC mass spectral analysis. Aggreceride showed no antimicrobial activity at 1000 µg/ml (paper disc method against yeast, fungi, bacteria) and had an acute toxicity (LD50) >200 mg/kg in mice intravenously. Aggreceride A inhibited thrombin-induced aggregation of washed platelets by 92% and 81% at 50 µg/ml and 25 µg/ml, respectively; it also inhibited aggregation induced by ADP, arachidonic acid, and PAF but was less active against collagen, with similar effects for B and C. Mechanistic studies via fluorimetric measurement of malondialdehyde (MDA) formation showed complete inhibition at 50 µg/ml of aggreceride A, suggesting action at the level of arachidonic acid metabolism or earlier. Notably, aggreceride is a 1-monoglyceride with platelet aggregation inhibitory activity, and further studies on its mechanism of action and pharmacological effects are ongoing.