The involvement of Ureaplasma urealyticum in male infertility has been suggested in 10 studies in which strains were found more often in the semen of infertile than fertile men and/or were associated with poor motility or other abnormalities. In contrast these findings have not been supported by those of an equal number of other studies. Ureaplasmas adhere to sperm and therefore have an opportunity to affect them. The numbers of organisms present in semen samples suggest that such a direct effect is unlikely but the question of whether metabolic byproducts affect sperm and whether ureaplasmas cause structural damage to the male genital tract remains unanswered. Ureaplasmas have been isolated from the genital tracts of women of infertile couples more often than from those of women of fertile couples by some but not all investigators and antibiotic therapy has also provided conflicting results. Two groups of workers have reported improvements in sperm motility and quality and increased conception rates in couples rendered Ureaplasma-negative by tetracycline administration. Such observations are provocative but have to be viewed against the failure of other investigators to note similar improvements in conception rates in studies where some attention was paid to controlling them. It should be noted, however, that the beneficial effect of an antibiotic may not be seen clearly if ureaplasmas are responsible for only a small subset of infertility cases.