Neutrophils contribute t o the pathophysiology of various ischaemic states. Since many agents thought t o be antiplatelet have also been shown t o affect neutrophil function, it was of interest to examine the effect of ticlopidine (250 mg, p.0.. b.i.d. for three doses), an antiplatelet agent, on f M LP (formyl-methionyl-leucyl-phenylalanine) stimulated neutrophil aggregation and luminol-dependent chemiluminescence in whole blood. Neutrophil aggregation did not significantly change from baseline values during ticlopidine administration. However, luminol-dependent chemiluminescence, an index of respiratory burst metabolism, was noted t o be markedly increased during ticlopidine administration. Two hours following the final dose of ticlopidine, the chemiluminescent response (mean f SEM, n = 5) was significantly increased from 6.27 f 1.88 t o 12.66 f 2.19 units ( p < 0.05). A return t o baseline (6.68 f 2.24 units) five days following the administration of ticlopidine was noted. It is concluded from this study that the acute oral administration of ticlopidine may affect neutrophil function as demonstrated by the significant increase in stimulated luminol-dependent chemiluminescence.