The proliferative behavior of neuroblastoma cells in bone marrow was evaluated before and after perturbation by chemotherapy. The data presented indicated that the proliferating fraction of the tumor cells before therapy was small. In most of the patients, initial therapy with a non-cell-cycle specific agent increased the mitotic and labeling indices. If initial therapy was followed by a cell cycle-specific agent, variable kinetic responses were observed. Although the number of patients was small and the follow-up was short, there was a correlation between the clinical response and the kinetic changes induced by the chemotherapy. Evaluation of cell kinetic changes during the initial course of chemotherapy might identify early those patients who are unlikely to have a good clinical response. Similar studies in the evaluation of new drugs to aid in identifying agents that favorably change cell kinetics and thus therapy for children with neuroblastoma was urged.