Nine inbred strains of mice have been studied in order to determine whether significant genetic influence is exerted on the radiosensitivity of the endogenous colony-forming unit (CFU), and whether a significant correlation exists between CFU survival and survival of the whole animal. Significant genetic control of CFU radiosensitivity has been demonstrated, but the number of CFU which manifest themselves in the spleen after exposure do not predict the probability of survival in an inter-strain comparison. Preliminary experiments indicate that additional factors, such as colony growth rates, colony differentiation, and strain differences in the requirement for regenerating hematopoietic tissue, must also be taken into account in an attempt to describe mechanisms of radiation resistance among inbred strains.