To determine whether there is an inherited familiar trait linked to the lymphocyte blastogenesis test (LTT), under stimulation with PHA, lepromin and Mycobacterium leprae in culture medium containing autologous plasma, this test was carried out in patients with the polar forms of leprosy and their parents. The lepromin reaction was also studied in the patients and their parents because, since the test is negative in lepromatous (L) patients and a greater proportion of negativity is detected among their relatives, it might be assumed that the lymphocytes of these individuals could have a lower tendency towards blastogenesis than lymphocytes of tuberculoid (T) patients and their relatives. Thirty individuals were studied, ten of them being leprosy patients (4 L and 6 T) and the remaining their parents; 115 LTT, including control and stimulated cultures, were performed. In the limited number of patients and parents studied, the results showed that mothers of either L or T patients displayed a similarly low response to the stimulants M. leprae and lepromin. The lepromin reaction was negative in all L patients and positive in 3 out of 8 parents, as well in all the T cases and their fathers. Fathers and their T descendants were lepromin positive and there was a certain relationship between this reactivity and blastogenesis. This might suggest a possible inherited familiar trait related to a relative degree of resistance. However, further evidences from studies with larger number of subjects are required to support this hypothesis.