ABSTRACT: Repeated semen deposition in the gut may be linked to the development of viral infections in homosexual men. Other investigators have suggested that rectal insemination may diminish immune responsiveness. We approximated conditions of human insemination by infusing 2 ml of pooled human seminal plasma (SP) into the rectum and/or vagina of rhesus monkeys. This resulted in increased blood plasma concentrations of the bicycloderivative of prostaglandin E (PGEM-II) which reached peak concentrations 2 h after rectal SP instillation in seven of eight test monkeys, but not the controls. The rate of PGE2 diffusion appeared to occur more rapidly across vaginal than rectal mucosa. Suppression of peripheral cellular immune functions was not demonstrated after the single exposure of this study, although persistent and repeat exposures could lead to local or generalized suppression of host defense mechanisms. Absorption of PGE's from the gut may be a cofactor in the development of sexually transmitted viral diseases.