Our objectives were to investigate the mechanisms of postbreeding inflammation in swine by examining the chemotactic properties of polymorphonuclear neutrophilic granulocytes (PMN) and of various populations of spermatozoa and seminal plasma. Epididymal spermatozoa from two boars obtained under sterile conditions, washed ejaculated spermatozoa from two boars, and pooled seminal plasma from eight boars of known fertility were examined for chemotaxis to PMN. The chemotaxis of blood-derived PMN in response to sperm and seminal plasma was evaluated and expressed as a percentage of a positive control (lipopolysaccharide-activated blood plasma). The mean chemotactic effect of washed sperm alone (4.4+/-0.04) and of epididymal sperm alone (3.4+/-0.06) was not different from that of the negative controls (3.1+/-0.05) of McCoy's medium with 10% heat-inactivated fetal calf serum. A marked chemotactic effect was detected when washed ejaculated and epididymal sperm were incubated with blood plasma, compared with blood plasma without spermatozoa (P < 0.001). Washed sperm in blood plasma (86.2+/-5.6) and epididymal sperm in blood plasma (83.9+/-7.7) were different from blood plasma alone (11.2+/-1.5), but no differences were detected between the two populations of sperm. This effect, however, was not completely inhibited by heat inactivation of the blood plasma. The chemotactic response of washed ejaculated and epididymal spermatozoa incubated in lipopolysaccharide-treated, heat-inactivated blood plasma were greater than that of the negative control (P < 0.05). Polymorphonuclear neutrophilic granulocyte migration toward seminal plasma was similar to the negative control (4.0+/-0.04 vs 3.1+/-0.05). It seems that porcine epididymal sperm and ejaculated sperm activate chemotactic components in porcine blood plasma and heat-inactivated blood plasma, suggesting that, at least partially, a heat-stable (noncomplement) blood plasma component may be involved in sperm-induced PMN chemotaxis. In contrast, porcine seminal plasma was not chemotactic to PMN. These results support the hypothesis that spermatozoa play an active role in initiating postbreeding endometritis.