The influence of material released by activated human platelets on the phagocytic activity of human neutrophils and monocytes has been investigated under conditions where there was a low background level of exposure of the leukocytes to platelet release products (PRPr). Incubation with PRPr prepared from 400 X 10(9)/l platelets resulted in an approximate 3-fold increase in the proportion of leukocytes which ingested particles. Stimulation was produced by dilutions of PRPr down to 1:4 to indicate that phagocytic activity was affected by material released by concentrations of platelets within the physiological range. This response appeared to reflect stimulation of the overall phagocytic capability of the leukocytes as equivalent ingestion was observed with particles which possessed or lacked the capacity to bind complement to their surface. Enhancement of phagocytic activity was evidently secondary to induction of a change in leukocyte behaviour which was sustained in the absence of continued exposure to preparations of PRPr. These observations extend the range of human biological processes known to be responsive to PRPr, and have implications for the assessment of leukocyte phagocytic activity in vitro as this particular property could be modified by contact with PRPr during the isolation of leukocytes from peripheral blood.