A battery of laboratory tests was applied in the study of a family with chronic lymphocytic leukemia in three sibs, the progeny of a consanguineous mating. Two survivors with leukemia and a sib showed impaired in vitro lymphocyte transformation in response to phytohemagglutinin and selective deficiency of serum immunoglobulin levels. Two other sibs showed only selective immunoglobulin deficiency. The array of findings suggests that a genetically controlled immune mechanism is involved in familial clusters of chronic lymphocytic leukemia.