The IF-producing capacity of human lymphocytes obtained by drainage of thoracic duct of surgical patients was studied. The lymph cells incubated with the appropriate inducer produced interferon of alpha and gamma types. The level of IF production, particularly of the gamma type, was negatively influenced by the storage of lymph at +4 degrees C and recovery of the cells in ficoll-hypaque gradient. The time course of IF production was found on the whole to be similar to that in the peripheral blood lymphocytes. The level of alpha-IF synthesis was higher for blood cells, and that of gamma-IF for lymph cells. The synthesizing capacity of the thoracic duct lymphocytes decreased in the acute stage of the disease and increased during convalescence which was especially typical of gamma-IF production.