Summary
Human platelet factor 4 (PF-4) showed a reaction of complete identity with PF-4 from
Macaca mulatta when tested against rabbit anti-human-PF-4. Such immunoglobulin was used for quantitative
precipitation of in vivo labelled PF-4 in monkey serum. The results suggest that the active protein had an
intra-platelet half-life of about 21 hours. In vitro 125I-labelled human PF-4 was injected intravenously into two monkeys and isolated by
immuno-precipita-tion from platelet-poor plasma and from platelets disrupted after
gel-filtration. Plasma PF-4 was found to have a half-life of 7 to 11 hours. Some of
the labelled PF-4 was associated with platelets and this fraction had a rapid initial
disappearance rate and a subsequent half-life close to that of plasma PF-4. The results
are compatible with the hypothesis that granular PF-4 belongs to a separate compartment,
whereas membrane-bound PF-4 and plasma PF-4 may interchange.