A protein fraction extracted from the lysosomal granules of human platelets generated chemotactic activity for polymorphonuclear leukocytes when incubated with fresh serum. The platelet factor was also released during platelet aggregation with collagen or epinephrine and appeared to be released during blood clotting. Heated serum did not support the platelet-dependent generation of chemotactic activity. Treatment of fresh serum with antibody to the fifth component of complement also prevented development of activity. Purified human C5 but not C3 yielded chemotactic activity upon incubation with the platelet factor. Thus, human platelets are capable of stimulating chemotaxis via complement activation in a manner similar to leukocytes, and may therefore participate in the early stages of inflammation.

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