1 The effect of the chemotactic peptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) was studied on cells in whole rabbit blood or on a mixture of purified rabbit platelets and neutrophils. 2 In blood, FMLP triggered cell aggregation (measured by electrical impedance) which was dependent upon the concentration of FMLP (9.9 +/- 0.7 and 5.2 +/- 1.2 ohms at 1 and 0.01 microM FMLP respectively). This aggregation was accompanied by a strong decrease in platelet counts (54.6 +/- 6.0 and 45.6 +/- 3.8% for 1 and 0.01 microM FMLP respectively) and by a smaller decrease in neutrophil counts (25.0 +/- 1.9 and 12.9 +/- 1.7% at 1 and 0.01 microM FMLP respectively). 3 When purified platelets and neutrophils were co-incubated, the addition of 0.1 microM induced a marked aggregation (50.0 +/- 1.6 vs. 19.5 +/- 1.6% of light transmission, n = 8, P less than 0.001), ATP secretion (8.4 +/- 1.0 vs. 0.1 +/- 0.1 nmol ml-1, n = 6, P less than 0.001) and a decrease in platelet counts. FMLP induced aggregation of purified neutrophils and release of lysozyme but lacked direct platelet-stimulating effects. The release of lactate dehydrogenase, a cytoplasmic marker and lysozyme were unchanged under the interaction conditions. 4 Platelet activation was reduced by about 30% with 100 microM aspirin or indomethacin and by about 70% with 100 microM BW 755C. Two Paf-acether antagonists, BN 52021 (100 microM) and WEB 2086 (1 microM) suppressed platelet activation by 70-80%. 5 The supernatant of FMLP-stimulated neutrophils induced platelet activation only when bovine serum albumin was present. Rabbit neutrophils stimulated in the presence of serum albumin by 1 microM FMLP formed 2 nM Paf-acether of which half was released to the extracellular medium. 6 Our results indicate that the stimulation of neutrophils by FMLP induces platelet activation in whole blood and on isolated cells and that both arachidonic acid-metabolites and Paf-acether participate in platelet activation.