An immunoalkaline phosphatase technique was used to examine the lymphoid cells in serous effusions from five patients with malignant lymphoma. The results were interpreted along with the morphologic studies and retrospective assessments of the clinical conditions of the patients. Two patients had no involvement of the serous cavities, and two had proven involvement. The fifth patient was studied while his lymphoma was evolving from inapparent to disseminated disease. In the two patients without involvement of the serous cavities, the effusion lymphocytes were predominantly monoclonal T cells, comparable to those in six patients with diseases other than lymphoma. In those with involvement of the serous cavities, the effusion lymphocytes were predominantly monoclonal B cells. In the patient with lymphoma in evolution, immunocytochemical studies accurately reflected the progression of disease. We conclude that immunocytochemical studies of the lymphocytes in serous effusions help not only to differentiate reactive from neoplastic lymphoproliferation but also to assess the status of lymphomatous involvement of the serous cavities. The immunocytochemical studies are most effective when correlated with clinical and cytologic studies.