Fungi of six families, encompassing 28 species, have been screened for their inhibitory effects on adenosine-5'-diphosphate (ADP), acetyl-glyceryl-phosphorylcholine (PAF) or collagen-induced rabbit platelet aggregation. Some fungi have also been studied for their ability to inhibit neutrophil aggregation induced by calcium ionophore A 23187. The results suggest that fungi may be a potential source of inhibitors of platelet and neutrophil aggregation.