✓ Since the development of a specific monoclonal antibody against the thymidine analogue bromodeoxyuridine (BUdR), many investigators have used intravenous infusion of BUdR to estimate the proliferative potential of human brain tumors. However, side effects such as the induction of cell mutation, latent virus promotion, or inhibition of cytodifferentiation cannot be ignored, and thus many workers hesitate to use it in patients, especially those with hepatic disease or of reproductive age. Furthermore, if BUdR remains in the deoxyribonucleic acid of tumor cells after injection, analysis of the effect of chemical and radiation therapy may not be evaluated correctly. In this report, in vitro BUdR labeling with an anti-BUdR antibody is compared with the in vivo methods described by previous authors. This method appears to be useful for determining the S-phase fraction of human brain tumor. It was more rapid, and was simple, safe, and reproducible as compared to the intravenous infusion method.