Ki-67 expression in the seminiferous tubule of the bovine testis was studied by immunohistochemistry during the seminiferous epithelial cycle using the monoclonal antibody MIB-1. Spermatogonial proliferation is most obvious in stages 5-7, and 8, when B-spermatogonia divide. A lower rate of spermatogonial propagation is observed preceding or during meiosis in stages 1-4. The MIB-1 antibody also gives positive results with some post-spermatogonial tubular cells. Preleptotenes passing through S-phase in stage 1 reveal positive nuclei. During prophase of meiosis I pachytenes react strongly, diplotenes react in an attenuated manner, while leptotenes and zygotenes stay negative. Secondary spermatocytes seen in stage 4 are positive as are the chromosomes during meta- and anaphase of the meiotic divisions. Post-meiotic spermatids are also decorated but stop Ki-67 expression abruptly at the end of stage 4. Sertoli cells are negative.