Platelet-aggregating factor (PAF) was removed from bovine plasma by human platelets fixed with 2% formaldhyde. The degree of adsorption was directly related to the platelet concentration and the length of incubation. Fixed washed platelets (FWP) aggregated with bovine plasma could be deaggregated by 1M KCl, Evans blue and 8M urea but not by .beta.-galactosidase. Incubation with 1M KCl eluted some but not all of the PAF, as the deaggregated platelets spontaneously aggregated upon removal of the deaggregating conditions. Also, fixed platelets adsorbed PAF even in the presence of 1M salt or after treatment with Evans blue. Platelet aggregation was not affected by thrombin (20 .mu./ml) but was abolished by trypsin at concentrations as low as 4 .times. 10-1 .mu.g/ml. Deaggregation is apparently not the result of elution of the loosely bound aggregating factor from the platelet surface, but rather the disruption of noncovalent interplatelet bridging between 1 or more PAF molecules bound to a specific receptor.