Although anti-platelet serum is frequently used to produce thrombocytopenia, the mechanism of how platelets are removed from the circulation is poorly understood. In vitro studies indicated that anti-platelet serum caused platelet aggregation, whereas normal rabbit serum did not. The aggregation was dose related; at the lower dose, aggregated platelets were shown to deaggregate and platelets released from the aggregates responded normally to ADP but not to collagen. In mice whose platelets were previously labeled with Na235SO4, anti-platelet serum caused an increase in the amount of 35S mucopolysaccharides (MPS) recovered from the kidneys. No increase in radioactivity was found in the lungs, livers, or spleens of similarly treated mice. The data suggest that platelet specific anti-serum causes platelets to aggregate leading to ADP (and consequently 35S-MPS) release; the accumulation of 35S in kidneys may be due to removal of MPS and/or platelets from the circulation.