The action of material released by activated human platelets on the phagocytic activity of human neutrophils and monocytes has been examined under conditions where there was a low background level of exposure of the leucocytes to platelet release products ( PRPr ). Incubation with PRPr produced an approximate three-fold stimulation of particle ingestion by both cell types. Material with stimulatory activity passed through ultrafiltration membranes capable of retaining molecules greater than 500 Daltons. This low molecular weight stimulator was inactivated by ATP diphosphopyrolase to suggest it could be one or more of the nucleotide di- or triphosphates in PRPr . Comparison with the action of specific nucleotides revealed only ADP and ATP exerted equivalent stimulatory effects at concentrations consistent with those of nucleotides normally present in PRPr . This effect was mediated by induction of a change in leucocyte behaviour which was sustained in the absence of continued exposure to the stimulatory agent. Stimulation of phagocytosis was also produced by high molecular weight material in PRPr , but this appeared to be mediated by dissociation of low molecular weight stimulator from a high molecular weight complex. These observations extend the range of biological functions known to respond to nucleotides in PRPr , and have implications for the interpretation of leucocyte phagocytic activity assessed in vitro in view of the potential modification of this process by exposure of leucocytes to PRPr during the isolation of leucocytes from peripheral blood.