Semen samples from 31 men, all of whom had fathered a child within the preceding year, were analysed for sperm characteristics and for the content of prostaglandins (PGs). Mean concentrations (mg/l) for the four main groups of PGs were 67.1 for PGE, 3.2 for PGF, 245.7 for 19-hydroxy-PGE and 13.3 for 19-hydroxy-PGF. The individual values were distributed over a relatively wide range but the extremely wide ranges reported by previous authors were not confirmed. Information is also presented concerning the relative proportions of the four isomers of PGF, as well as those of 19-hydroxy-PGF. Only sperm density was related to PG concentration in fertile men. Polyzoospermia was associated with a low PGE concentration.