We hypothesise that molecules in the cyclooxygenase pathway affect platelet activity when seminal fluid (SF) is present. We considered the influence of SF on platelet aggregation in women, and believe that the prostanoids in SF signalling are significant. Thirty‐one female subjects were studied, 20 of whom were sexually active. Male partners were given either aspirin or indomethacin to inhibit cyclooxygenase. The 6‐keto prostaglandin F1α (6‐keto PGF1α) and prostaglandin E metabolite (PGE‐M) in SF were measured by competitive assay. Platelets and prostanoids were evaluated in women, periodically, before and after intercourse. The platelets were tested with adenosine diphosphate (ADP) and arachidonic acid (AA). To block the interaction between the uterus and SF, some couples used condoms. We found that the 6‐keto prostaglandin F1α in urine at 2 hours post‐intercourse (1418.75 pg/mL, Std 688.39) was greater than pre‐intercourse (772.68 pg/mL, Std 116.54). Post‐intercourse, a transient decrease in platelet aggregation was observed in women whose partners did not use condoms. Averages for platelet aggregation were 20.16% with ADP, and more significantly, 37.79% with AA after 2 hours. In contrast, couples using condoms showed no changes, averaging 64.02% with ADP and 72.06% with AA. Women whose partners were taking aspirin or indomethacin also showed no changes. SF from men taking aspirin or indomethacin led to no reduction in platelet aggregometry in their partners. These results indicate that in cases of exposure to SF, the transient change in women's platelet activity could be related to the cyclooxygenase pathway.