Human platelets were loaded with aequorin, a Ca2(+)-sensitive photoprotein, and tested in the platelet-ionized calcium aggregometer for simultaneous recording of platelet aggregation and intraplatelet Ca2+ levels both in the presence and in the absence of autologous polymorphonuclear leukocytes. Cells were exposed to one of three chemotactic stimuli: platelet-activating factor (PAF), N-formyl-methionyl-leucyl-phenylalanine (FMLP), or leukotriene B4 (LTB4). Platelets alone aggregated and showed intracellular Ca2+ movement only when exposed to PAF. Amplification of both platelet aggregation and intraplatelet Ca2+ movement was induced by PAF in the presence of leukocytes. Aggregation and intraplatelet Ca2+ mobilization were also observed in the presence of leukocytes activated by either FMLP or LTB4. Both parameters increased with the concentration of the stimuli and/or the number of leukocytes. Platelet thromboxane B2 production was also significantly increased in the presence of leukocytes. Addition of platelets at different times after leukocyte activation resulted in progressively reduced cytoplasmic Ca2+ increase. Cell-free supernatants prepared from FMLP-stimulated leukocytes were able to induce platelet aggregation, thromboxane B2 generation, and Ca2+ mobilization, although at a reduced degree as compared with intact leukocyte addition. The activity of leukocyte supernatants was stable at 37 degrees C for up to 30 min and was suppressed by trypsin inhibitor. Our study indicates that stimulated leukocytes release a soluble enzymatic activity able to activate platelets; cell-to-cell interaction may also play a role in this phenomenon. Platelet-leukocyte interaction could have physiopathological relevance and constitutes a new model for studying old and new platelet inhibitory drugs.