The radiation sensitivity of fibroblasts isolated from bilateral retinoblastoma (RB) patients was investigated using an in vitro micronucleus assay. Bilateral RB is an autosomal dominant disease associated with a single locus, RB-1; therefore, all cells in an affected individual carry the germ line mutation. The ability to identify gene carriers made it possible to study the effect of the RB-1 mutation in the heterozygous state on the sensitivity of the cells to chromosome breakage by gamma-rays. The micronucleus assay was chosen for this study since it is a quick and easy measure of chromosomal aberrations. The fibroblasts from bilateral RB patients did not differ systematically from the normal fibroblasts in either the spontaneous or the induced rates of micronucleus production. Thus, bilateral RB fibroblasts are not more sensitive to the clastogenic effects of gamma-radiation than the controls.