A monoclonal antibody, SN3, has been prepared against a cell membrane fraction of the pre-B leukemic cell line NALM-1. By radioimmunoassay, SN3 reacted with four of four non-T/non-B, two of two pre-B and one of three leukemic B cell lines. The reagent was unreactive, however, with established leukemic T and myelomonocytic cell lines or normal B cell lines. On immunohistochemical assays on frozen sections of nine reactive lymph nodes and three spleens, SN3 showed a preferential binding to 50-95 per cent mantle zone (MZ) cells and 5-20 per cent interfollicular or red pulp B-lymphocytes. This was uninhibited by pre-incubation with heterologous anti-HLA-DR or anti-delta reagents. SN3 was unreactive with normal germinal centre (GC), epidermal or Langerhans cells but did react with less than 1 per cent thymic B-lymphocytes. In eight follicular small cleaved cell lymphomas tested SN3 exhibited three patterns of reactivity: peripheral follicular, combined peripheral and central follicular, and combined follicular and interfollicular. Three follicular lymphomas were essentially SN3-. In three diffuse small lymphocytic lymphomas, SN3 showed patchy areas of reactivity unassociated with proliferation centres. In four diffuse B-cell lymphomas (one mixed small and large cell, two large non-cleaved cell, and one small non-cleaved (Burkitt) cell), SN3 reactivity was uniformly distributed in the majority (60-90 per cent) of the cells. SN3 was unreactive with one diffuse B-large cell lymphoma, three nodal T-cell lymphomas and three cases of mycosis fungoides. The findings indicate that SN3 detects an antigen that is present in subpopulations of normal MZ cells, the antigen is also detected in GC cells undergoing lymphomatous transformation but is not readily detected in normal GC cells, and the antigen is also expressed in subpopulations of diffuse B- but not T-cell lymphomas.