Cell kinetic studies on various human brain tumors were reviewed. Most studies have been carried out by means of 3H-thymidine and autoradiography in the past two decades. The average labeling index (LI) obtained from a pulse of 3H-thymidine is very high in medulloblastomas and glioblastoma multiforme (5-15%), low in well-differentiated gliomas (less than 1%), and intermediate in anaplastic astrocytomas. The higher the LI, the faster the tumor grows, probably reflecting a larger growth fraction. Therefore, measurement of the LI appears to be very helpful in predicting the prognosis of the patient as well as potentially helpful in the design of chemotherapy. However, isotopic studies not only take a long time to complete, but also have severe limitations because of potential radiation hazard to patients and environmental pollution. Development of an anti-BUdR (or BrdUrd) monoclonal antibody to detect nuclei which incorporate BUdR is a breakthrough that can expand this line of research, since: 1) BUdR is non-radioactive and doses needed for this purpose are virtually non-toxic; 2) use of flow cytometric analysis of FITC conjugated anti-BUdR antibody facilitates rapid acquisition and data analysis and, 3) it will be technically easier to study the proliferative capacity of patients with brain tumors. A 30 min exposure of 9L monolayer cells to 10 X 2-10 X 2(-4) microM BUdR produced satisfactory results against a 1: 60 dilution of FITC conjugated antibody (Becton-Dickinson, Mt. View, CA). Also 9L cells which were exposed various concentrations of BUdR (10 X 2-10 X 2(-4) microM) for 30 min were harvested in single cell suspension and reacted with the antibody and analysed with flow cytometry. Fluorescent nuclei (48.6%) were similar to the fraction of cells in S phase analysed from DNA histograms as well as the LI obtained from autoradiographic study after a pulse of 3H-thymidine. After 1-40 mg/kg of BUdR were injected into the peritoneal cavity of rats with 9L brain tumors, tumors were removed 1 hr later and digested with enzyme cocktail, fixed with 70% ETOH, denatured with HCl, and stained with FITC conjugated anti-BUdR antibody. The nuclei which reacted with the antibody were discriminated well with flow system at 488 nm light through LP 515 and SP 560 filters. 10.0-15.5% of the cells were fluorescent in each group and the intensity of fluorescence was dose dependent although not strictly proportional to the amount of BUdR administered.(ABSTRACT TRUNCATED AT 400 WORDS)