[en] Neoplasms of possible radiogenic origin developed in two members of a family prone to a diversity of cancers. Gamma-irradiation survival studies in these two patients and three other relatives, but not their spouses, over three generations demonstrated resistance to cell killing. The D₁₀ value (radiation dose required to reduce survival to 10%) was significantly higher for the five radioresistant strains (491 +- 30 rad) than for control cultures (405 +- 18 rad). There was a significant correlation between individual D₁₀ values and D₀ survival-curve parameters, indicating that changes in the exponential slope of the survival curves accounted for much of the increase in D₁₀ values. This novel radiation phenotype could be a manifestation of a basic cellular defect, predisposing to a variety of tumours in family members. Thus in-vitro radioresistance, like radiosensitivity, may be a phenotype of a mechanism that increases cancer risk in man. (author)