Major changes in DNA distributions of two human neuroblastoma cell lines growing in vitro and in athymic nude mice occurred after treatment with cyclophosphamide. Pulse treatment of LA-N-1 cells in vitro with liver S-9-activated cyclophosphamide (10 micrograms/ml) caused approximately 50% cytotoxicity; flow microfluorometric analysis of surviving cells demonstrated an increased proportion of G2 + M cells and a decreased proportion of G1 cells, particularly at 48 hrs. Even though LA-N-1 tumors in nude mice did not regress after one dose of cyclophosphamide (250 mg/kg), the percent of G2 + M cells increased and the percent of G1 cells decreased 4-6 days after treatment; the percent of cells in S increased at 2 and again at 8 days. SK-N-MC cells were affected differently by cyclophosphamide. Pulse treatment of these cells in vitro with liver S-9-activated cyclophosphamide caused greater than 85% cytotoxicity and nearly complete elimination of cells in G2 + M at 24 and 48 hrs. Likewise, SK-N-MC tumors in nude mice regressed greater than 50% after cyclophosphamide, and the proportion of G2 + M cells decreased markedly 2-6 days after therapy. We conclude that cyclophosphamide can have different cytotoxic and cytokinetic effects on neuroblastomas. In addition, marked cytokinetic effects may occur even though changes in tumor size are minimal or not detectable.