The distribution pattern of proliferating cell nuclear antigen (PCNA) and Ki-67 protein was studied in adult bovine seminiferous epithelium by means of immunohistochemistry using monoclonal antibodies. Tailoring the methodological protocol for each of the two proliferation markers was a necessary prerequisite for obtaining optimal results in tubular sections and whole-mounts. A-, I- and B-spermatogonia displayed PCNA-positive nuclei, except during meta-, ana- and telophases of mitosis. PCNA-negative nuclei in the basal tubular compartment, excluding those from non-cycling Sertoli cells, belonged to the spermatogonia precursor cell line. However, only about 30%, 45% and 47% of the respective A-, I-, B-spermatogonia had positive nuclei after exposure to the MIB-1 antibody directed against the Ki-67 protein. Spermatogonia with MIB-1-negative nuclei represented cells in the G1-phase. Both antibodies reacted intensely with the nuclei of preleptotene primary spermatocytes. PCNA reactivity was also present during leptotene through pachytene. Ki-67 protein expression was absent during leptotene and zygotene but was again encountered during pachytene and meiosis I and II. Anti-PCNA/anti-protein gene product 9.5 double-labelling indicated that the transition from spermatogonia precursor cells into A1-spermatogonia is not strictly synchronized in a given tubular segment, a possible reason for the flexibility in A-spermatogonial propagation seen in bovine seminiferous tubules.