The quantitative distribution pattern of Ki-67 protein and proliferating cell nuclear antigen (PCNA) immunoreactivity was studied in human testis biopsies. In normal seminiferous epithelium Ki-67 is expressed in nuclei of spermatogonia, while PCNA additionally occurs in nuclei of primary spermatocytes. The staining pattern of spermatogonia is as follows (Ki-67-positive/PCNA-positive): 26.6 +/- 12.4%/46.3 +/- 9.5%. No stage-dependent differences were found. Biopsies with mixed atrophy (score < or =7) showed a significant (P < 0.05) decrease of immunopositive spermatogonia to 19.9 +/- 3.0%/31.4 +/- 5.7% (score 1) with minimal variation between different samples (score 7 to 1). Associated with defined histological defects such as hypospermatogenesis (hyp), spermatogenic arrest at the level of spermatids (sda), spermatocytes (sca) or spermatogonia (sga), however, there was a significant (P < 0.05) decrease of Ki-67 staining in tubules showing hyp (28.6 +/- 8.8%), sda (25.6 +/- 9.3%), sca (23.7 +/- 9.3%) and sga (16.2 +/- 6.0%) and of PCNA staining in sca (32.2 +/- 11.8%) and sga (20.0 +/- 9.5%), respectively. The decrease of immunoreactive spermatogonia did not correspond to elevation of follicle stimulating hormone (FSH). These data demonstrate that the low spermatogenic efficiency in infertile men is not only due to postmeiotic events, but also to a decrease in the meiotic activity of spermatogonia, and is not related to serum FSH.