OBJECTIVE The aim of this study was to examine the vasoactive effects of rabbit isolated mononuclear leukocytes, and to identify the mediators responsible for those vasoactive effects. METHODS Mononuclear leukocytes (MNLs) were isolated from New Zealand White rabbit whole blood, suspended at 5 x 10(7) cells/ml and incubated for 30 min at 37 degrees C. This cell suspension, or the cell-free supernatant from this suspension, were then examined for vasoactive effects in rabbit isolated thoracic aorta. RESULTS Both the MNL suspension and the cell-free supernatant from this suspension caused endothelium-independent contraction of aortic rings, both from resting tension and when pre-contracted. The MNL suspension caused a significantly greater contraction than the MNL supernatant under all conditions. The contractions to the MNL product were significantly inhibited by the 5-HT2 receptor antagonist ketanserin (0.1 microM), but not by the alpha 1-adrenoceptor antagonist prazosin (10 microM). High-performance liquid chromatography (HPLC) analysis showed that the MNL supernatant contained serotonin (5-hydroxytryptamine, 5-HT) at an average concentration of 5 microM. CONCLUSIONS We conclude that MNLs cause contraction of rabbit isolated aortic rings by the release of 5-HT.