Basophil-derived platelet activating factor (PAF) was shown to induce the release of platelet factor 4 (PF4), a specific plaelet protein, from isolated, washed rabbit platelets. The kinetics and dose response of this release were similar to that of PA-induced 3H-serotonin secretion, and indicated that PF4 release was a reliable and sensitive estimate of platelet stimulation y PAF. Plasma PF4 concentrations were therefore used as a marker for in vivo platelet release during systemic anaphylaxis inducd by the i.v. injection of antigen (horseradish peroxidase, HRP) into rabbits conditioned to produce only IgE antibody against RP. Antigen administration was accompanied by the loss of circulating stainable basophils, acute thrombocytopenia and neutropena, and a significant increase in PF4 levels. The increase in plasma PF4 began approximately 90 sec after antigen challenge, coresponding to 60 sec after the intravascular aggregation of platelets. However, maximum levels of plasma PF4 were not reached unil 3 to 5 min after antigen challenge. Concomitant measurement of circulating PAF during anaphylaxis indicated that PAF releaseinto the circulation preceded the decrease in circulating platelet numbers by about 30 sec and preceded the increase in plasma PF4 levels by about 60 sec. Thus, this study documents that platelets undergo a release reaction during anaphylaxis at a time hen they are sequestered in the microvasculature. The temporal relationship of this release suggests that the following series f events occur during IgE anaphylaxis: antigen stimulates circulating IgE-sensitized basophils to release PAF, PAF causes intraascular platelet aggregation and subsequent sequestration, and finally, sequestered platelets release their intracellular consttuents.