Skin fibroblasts from patients with hereditary retinoblastoma (RB cells) were examined since predisposition to the tumour might be expected to involve some degree of chromosomal instability, as has been noted for several cancer-prone conditions. Spontaneous and N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced aberration frequencies were measured, the cytogenetic effects of long-term treatment with 12-O-tetradecanoyl-phorbol-13acetate (TPA) were examined and also the spontaneous and TPA-induced sister chromatid exchange (SCE) frequencies. In all the studies the RB cells behaved in a similar fashion to normal human skin fibroblasts.