Skin fibroblasts from normal children and two children with a 13q14 deletion retinoblastoma (Rb) were submitted to fractionated doses of gamma radiations. Irradiation reduced the population doublings in normal fibroblasts and the decline was inversely related to the dose. An increase in population doublings was obtained with one of the Rb cell lines. Foci appeared in the irradiated culture of the other Rb donor. It is suggested that fibroblasts from patients with Rb are able to express some phenotypical properties of transformed cells, perhaps related to factors rendering them more susceptible to carcinogens.