Seven patients, long-term survivors of Hodgkin's disease, and 24 of their relatives (parents, siblings and children), together with normal controls were studied for percentages, absolute counts and mitogen-proliferative responses by means of monoclonal antibodies, E rosette technique and in vitro cultures with PHA, ConA and PWM. The aim of the study was to ascertain whether the impaired cell-mediated immunity of Hodgkin's patients was also present in relatives in order to elucidate the still debated etiology of the defect and of the disease (congenital? environmental? infectious?). The results show that both Hodgkin's patients and their relatives have a significant decrease of total T cells (as T3+, T11+ and E rosette-forming cells) in peripheral blood and a significant impairment of polyclonal responses to all the mitogens employed. The Leu-7+ cells (i.e. a consistent amount of natural killer cells) are significantly increased only in the Hodgkin's patients but not in their relatives. The T cell subpopulations (T4 and T8), B cells and monocytes do not show any difference between the patients, their relatives and normal controls. Our results seem to support, at least in part, the presence of a common defect of T cell lineage both in patients and in their relatives, but its etiology still remains uncertain (genetic? environmental?).