Loss of surface P-selectin from platelets. Platelets were activated (Activated) with mouse thrombin (1 U/mL) and incubated for indicated times under defined conditions. The percent of P-selectin–positive platelets was then evaluated by FACS analysis. The incubation of platelets up to 4 hours in the resuspension buffer (pH 7.3) (Buffer) or platelet-poor plasma (PPP) did not significantly alter the expression of P-selectin on the activated platelets, whereas reinjection (Reinjection) after biotinylation into an animal or incubation at room temperature with whole blood (Blood) induced a dramatic decrease in plasma membrane P-selectin. Incubation of resting platelets (Resting) in the resuspension buffer did not upregulate P-selectin expression.