Cellular immunity in lung cancer was studied by means of lymphocyte transformation by phytohemagglutinin (PHA), purified protein derivative (PPD) skin test, absolute counts of peripheral lymphocytes and leucocyte migration inhibition test. The lymphocyte transformation by PHA was reduced in lung cancer patients in comparison with healthy controls, and correlated with the disease stage, but not with the histology type. The lymphocyte transformation became further depressed by cancer chemotherapy. PPD skin reaction or absolute counts of peripheral lymphocytes did not correlate with the disease stage. However, all the three tests correlated with the disease stage. Therefore, these tests appeared to be general indices of the immunologic status. Inhibition of leucocyte migration was demonstrated in about a half of the patients with lung cancer, but not in patients with other cancers or infectious lung diseases or in healthy controls. These findings suggest the presence of tumor-associated antigens in lung cancer. Concerning the effect of the streptococcal agent, OK-432, enhancement of the function of lymphocyte was observed in the parameters such as lymphocyte transformation by PHA, PPD skin test and leucocyte migration inhibition test. In addition, OK-432 yielded a longer survival in lung cancer patients when used in combination with chemotherapy than with chemotherapy alone.