Patients with melanoma who had one or more close relatives with melanoma were studied for their natural-killer-cell (NK) activity against cultured melanoma cells and Chang cells. A high proportion of the patients and their relatives were found to have low NK activity against these target cells. In most of the patients this could not be attributed to general depression of their immune function, since B- and T-cell numbers and the mitogenic response to PHA were within normal limits. The levels of NK activity of the patients and their relatives were found to be significantly correlated, suggesting that the NK activity in these families may have been genetically (or environmentally) determined. Several genetic markers were examined in the patients and their relatives for association with the disease state and NK activity. No association with HLA antigens or ABO blood groups was detected, but there was a low incidence of the Rhesus negative phenotype in the patients (the Rh phenotype had previously been associated with high NK activity). The present results indicate that NK activity has a familial association in families with a high incidence of melanoma, and raise the question whether low NK activity may be one of the predisposing factors in the development of familial melanoma.