Abstract Seven normal human subjects showed acute, reversible thrombocytopenia, after administration of intravenous heparin. The phenomenon was repeated within 1 hour after return to normal (1 subject) and within 1 week (1 subject). There was good coincidence between initiation, time of maximal effect and total duration of the thrombocytopenia, and the prolongation of the whole blood clotting times. In general, onset and duration of maximal thrombocytopenia usually lagged minutes behind the Lee-White time. In the one subject tested, neutralization of the heparin with protamine resulted in prompt return to normal level of the Lee-White time and a more rapid return to normal of platelet count. Here, also, platelet return lagged behind the Lee-White return. The acute thrombocytopenia could be reversed by neutralization of heparin and also could be immediately reinduced a second time. The data suggest that transient sequestration, rather than destruction and replacement of platelets, is the mechanism of the effect.