B and T cell populations were studied in blood and neoplastic tissues from 64 untreated and 23 treated patients with non-Hodgkin's lymphoma. This study was undertaken primarily to evaluate the relation of B and T cell markers in various lymphomas to the currently accepted morphologic classifications and to determine the utility of various tissues in defining the cell of origin of a lymphoma. When histologically involved blood, bone marrow, lymph nodes or body fluids were studied, a B or T cell origin of the lymphoma was identified in 26 of 28 (68 per cent) patients. A B cell origin was found in 17 adults classified as having nodular (N) or diffuse (D) poorly differentiated lymphocytic lymphoma (PDLL). One lymphoma of T cell origin was observed in an adult with poorly differentiated lymphocytic lymphoma-diffuse (PDLL-D). In contrast, all cases of PDLL-D in children were T cell in origin. The origin of American Burkitt's (stem cell) lymphoma in two children was the B cell. When histologically involved blood was studied, a B or T cell origin was demonstrated in 10 of 21 (48 percent) adults. Evidence of a monoclonal proliferation of B lymphocytes in the blood was found two adults with more than 7 per cent lymphoma cells in Wright-Giemsa stained blood smears. When neoplastic lymph nodes were studied, the diagnosis of a B cell lymphoma was made in 8 of 12 (67 per cent) adults. Study of surface markers on malignant cells in cerebrospinal or serosal fluids frequently revealed a B or T cell origin of the lymphoma. B and T lymphocyte numbers in the blood did not correlate with immunoglobulin or skin test abnormalities. Abnormalities in circulating B or T cell percentages at diagnosis were a poor prognostic sign in patients with PDLL-D.