Tumor cells from two patients with very different clinical diseases were evaluated using immunologic techniques. The first patient had a persistent T-cell lymphocytosis and other symptoms suggestive of T-cell malignancy. She also had an adnexal mass. Following surgical excision, the mass was identified as a benign teratoma that contained thymic tissue. The T-cell lymphocytosis subsided following removal of the mass. The second patient had leukemia, the cells of which were morphologically identified as myelomonocytic. Surface marker studies, however, showed the presence of monoclonal surface immunoglobulins. Enzymatic digestion of the cells showed that the antibodies were cytophilic and not produced by the tumor population. These studies emphasize the value of immunologic techniques in characterizing tumor cells.