The presence of T-mycoplasmas in semen specimens from 625 men with infertility of unknown etiology was correlated with seminal cytologic findings. A change in the distribution toward higher ejaculate volumes and lower counts was demonstrated in the 246 patients with positive T-mycoplasma cultures, compared with the 379 patients with negative cultures. There was a significant increase in the number of tapering forms and spermatids in the T-mycoplasma-positive group. The increases in aberrant forms occurred at the expense of the oval and microcephalic forms. This change in the cytologic picture is consistent with the "stress" pattern seen in viral infections and allergic reactions, with the exception of the specific decrease in microcephalic forms. Relatively fewer patients with T-mycoplasmas in their semen had a high-level spermatozoal motility, compared with those lacking T-mycoplasmas. More than half again as many patients with T-mycoplasma infections had the poorest levels of spermatozoal motility. Infertile patients with T-mycoplasma infections had an over-all decrease in semen quality compared with those lacking demonstrable T-mycoplasmas.