A rapid propidium iodide staining method was used for analysis of single cell suspensions of bone marrow and tumor biopsies by flow microfluorometry. With this technique, information on the proliferative status of target tissues can be obtained within 10 min of sample removal. DNA histograms and labeling index of sequential bone marrow biopsies from a patient with stage IV diffuse lymphocytic leukemia and treated with 1-.beta.-D-arabinofuranosylcytosine infusion showed pronounced reduction in the percentage of cycling cells. Sequential tumor biopsies from a melanoma patient on methotrexate-citrovorum factor rescue therapy showed no changes. In sequential bone marrow biopsies of 3 patients on high dose methotrexate-citrovorum factor rescue, initial accumulation of cells in G1-S (day 1) was followed by a significant proliferative response (days 4-7) and return to pretherapy values. No recovery similar to that of the bone marrow was seen in tumor cells.