The cytotoxic activity of Hodgkin serum on some isolated T and B lymphocyte populations of 10 normal subjects, 10 patients with Hodgkin disease in the acute phase and 10 patients with Hodgkin disease but in clinical and biohumoral remission was studied. The effect of citotoxic serum variously affects the two different lymphocyte populations, particularly inhibiting LIF activity of T lymphocytes, while stimulating LIF activity of B lymphocytes. The possible causes of this phenomenon are set forth as well as their clinical implications.