Autoantibodies cause platelet clearance. Individual “end-of-study” plasma samples from selected mice were injected IV into individual syngeneic mice. Platelet counts measured 24 hours after injection were compared to preinjection counts and are expressed as a percentage of the preinjection count. Platelet levels in mice injected with plasma samples with no detectable autoantibodies (○) remained unaffected. Platelet levels in mice injected with plasma samples containing autoantibodies and associated with thrombocytopenia (♢) decreased significantly (P < .02). The bars depict the average of all animals in the group.