The local immune response to lung cancer was investigated by histologic and immunologic means. Distinctive patterns of stromal cellular reaction, characteristic for different histologic types of lung carcinoma, were recognized. The amount of cellular infiltration was highest in squamous cell carcinomas and lowest or nonexistent in oat cell carcinomas. Within the various histologic categories the well-differentiated tumors appeared to be accompanied by more reactive cells than the poorly differentiated ones; there was no relation between tumor necrosis and cellular infiltration. The plasma cells were distinctly associated with squamous cell carcinomas; their number in the stroma was proportionate to the degree of differentiation and the presence of keratin produced by the tumors. Eluates with a high content of immunoglobulins were recovered from pleural effusions and from solid lung carcinomas by dissociation of antigen-antibody complexes. These preparations reacted positively in indirect immunofluorescence tests with tissue cultures and with fresh suspensions of lung carcinoma cells, but not with tissue culture cells of most nonpulmonary tumors or with cell suspensions of normal adult and fetal lung. Similarly prepared fractions of noncarcinomatous pleural effusions did not react with lung cancer cells.