Candida guilliermondii (C.g.) occurs mostly in animals, but it can affect humans too. Using serological screening methods studies have been carried out in 57 (30 normozoospermic and 27 oligozoospermic) asymptomatic infertile men, of whom 15 (26.3%; 9 oligozoospermic and 6 normozoospermic) were found to have responded to Candida guilliermondii var. guilliermondii. Sperm parameters (cell count, motility, morphology) were determined and various tests (hypo-osmotic swelling test, bovine mucus penetration, swim-up and testosterone examinations) were also performed. In the C.g.-infected and non-infected groups no significant difference was observed in any of the parameters. Precipitation was performed with a special antigen and in the positive group agglutination titres ranged from 0 to 1/320. Comparing C.g. titres with sperm parameters, only the swim-up technique showed noticeable alteration (P = 0.003). One month following ketoconazole treatment, the patients showed no precipitation reactions and their agglutination titres were < 1/80. Our results suggest that for the diagnosis of male infertility a serologic screening for C.g. is essential. As C.g. can also be sexually transmitted, both the husband and wife must be treated, if infection occurs.