The etiology of asymptomatic genitourinary infection and its role in infertility are controversial. Normal semen has up to three polymorphonuclear leukocytes per high poser field. Investigators have found that in the presence of bacterial prostatitis, the pH of expressed prostatic secretions rose to 8.32 from a normal value of 7.3. In addition, they have demonstrated that as the pH of the prostatic fluid increases, the zinc levels decrease. The presence of asymptomatic genitourinary infection may therefore be suggested by alterations in seminal factors (i.e., pyosemia, reduced seminal zinc, and/or elevated seminal pH). In the present study, 57 males with unexplained infertility were identified as possible candidates who might be harboring an asymptomatic infection. Their altered seminal factors and culture results are herein reported. Urine and semen samples were obtained from these men, in whom pyosemia (5 to 10 or more daily white blood cells per high power field), reduced seminal zinc (less than 1 5 mg/100 ml), and/or elevated pH (greater than 7.7) had been demonstrated in earlier semen analyses. Twenty-seven patients (47 per cent) were shown to be colonized with Ureaplasma urealyticum. Urinalysis was unremarkable, and routine bacteriological testing on both urine and semen specimens consistently yielded no growth. Cultures for Chlamydia trachomatis failed to reveal the presence of this organism in the patients. Analysis of the mean seminal factors revealed a significant difference with regard to seminal pH (P < 0.005) between the group colonized with U. urealyticum and the uncolonized group. In the colonized group, 75 per cent of the samples had elevated pH (greater than 7.7), as compared to 28 per cent of the samples in the uncolonized group. Pyosemia was slightly more prominent in the colonized group (74 vs. 50 per cent). Both groups demonstrated low zinc levels. U. urealyticum exhibited a marked sensitivity to mino-cycline (91 per cent), a high resistance to erythromycin (95 per cent), and sensitivity to tetracycline of only 18 per cent. Urine samples obtained before semen collection were positive for U. urealyticum 90 per cent of the time. Urine samples collected from the wives of the male patients were analyzed, and 75 per cent were found to be similarly infected. Ten couples consented to treatment with minocycline. After the wives were carefully screened by their gynecologists to rule out the possibility of pregnancy, both husbands and wives received 100 mg of minocycline daily for 30 days. All cultures became negative, and there …