The mechanism of pokeweed mitogen (PWM) dependent decreased IgG production by blood lymphocytes from lung cancer patients was studied in comparison to control patients and blood donors. It has been shown that the depletion of monocytes has some influence on IgG synthesis but is not a decisive factor. Also, quantitative alterations in the CD4 and CD8 lymphocyte subsets do not significantly influence the PWM stimulation index for IgG synthesis. The assessment of T lymphocyte suppressor activity in lung cancer patients was performed by means of a co-culture with blood mononuclear cells, while helper activity was evaluated through co-culture with donor B lymphocytes. It has been found that lung cancer patient T lymphocytes have no increased suppressor activity, however, especially in the CD4 subset, display the decrease of helper function for B lymphocytes in PWM-induced IgG synthesis. The weakened helper function of CD4 lymphocytes may explain the suppression of specific antibody synthesis do novo which is evident in patients with lung cancer.