Prostaglandin (PG) E and 19-OH PGE, now considered to be the most important of the human seminal prostaglandins, were assayed in infertile and normal men. In the 15 volunteers PGE and 19-OH PGE levels were 23-89 microgram/ml, respectively. In the 4 groups of infertile patients in whom either PGE or 19-OH PGE levels were increased or decreased with respect to normal, sperm concentration and motility were significantly reduced. The negative effects of low levels of seminal prostaglandins on sperm concentration and motility might be correlated respectively with decreased adenylcyclase and testicular androgen activity. The negative effects of high levels of prostaglandins on the seminal fluid might be due either to an inhibition in testicular DNA synthesis or to a decreased sensitivity of the receptors to high titers of prostaglandins.