To gain more insight into a possible detrimental influence of polymorphonuclear granulocytes (PMN) on fertility, PMN were added to spermatozoa and oocytes in the hamster oocyte assay. Addition of 2.5 x 10(6)/ml PMN to the sperm suspension at the start of the first incubation period (capacitation period) resulted in a significant decrease (P less than 0.03) in the number of decondensed sperm-heads per hamster oocyte (the decondensation index). When a concentration of 0.5 x 10(6)/ml PMN was used, no decrease was found. Stimulation of PMN by phorbol myristate acetate (PMA) did not enhance the inhibitory action of PMN. After incubation of oocytes with PMN for 1 h, an almost total inhibition of decondensation was observed. A significant inhibition could also be obtained when unstimulated PMN (2.5 x 10(6] were added at the start often 3 h coincubation period of spermatozoa and oocytes (P less than 0.004). The inhibition by PMN decreased to a nonsignificant level when the hydroxyl radical scavenger thiourea was present. The possible mechanisms by which PMN affect the results of the hamster oocyte assay and the possible implications of the presence of PMN in the genital tract are discussed.