ABSTRACT
Blood and pleural effusion mononuclear cells from thirteen patients were examined for the expression of T lymphocyte differentiation antigens as well as in vitro thymidine incorporation. The ratio of T4 to T8 cells was significantly greater among pleural effusion lymphocytes than among blood lymphocytes. Effusion lymphocyte responses to phytohaemagglutinin were less than those of blood lymphocytes. Unstimulated thymidine incorporation was greater in pleural effusion lymphocytes. Antigen-stimulated lymphocyte reactivity was not consistently greater in either blood or effusion lymphocytes. Lymphocytes from tuberculous effusions all reacted to tuberculin. Pleural effusion lymphocytes, regardless of the etiology of the effusion, possessed the same range of antigenic specificities as did blood lymphocytes. Therefore, effusion lymphocyte responsiveness to tuberculin does not prove the presence of tuberculous pleurisy but does indicate sensitisation to tuberculin.