NALM-6-M1 is an acute lymphoblastic leukemia cell line previously shown in our laboratory to express the pre-B lymphocyte phenotype, i.e., cytoplasmic IgM+, surface immunoglobulin-. Hybridomas were produced against this cell line by fusing spleen cells from hyperimmunized mice with NS-1 mouse myeloma cells. One monoclonal antibody derived from this fusion, designated BA-1, reacted with peripheral blood B lymphocytes, chronic lymphocytic leukemias, pre-B-ALL, most non-Hodgkin's lymphomas, and most non-T, non-B-ALL. BA-1 showed weak reactivity with B lymphoblastoid cell lines and failed to react with multiple myeloma and pokeweed mitogen-induced plasma cells. BA-1 also reacted with peripheral blood granulocytes and the erythroleukemia cell line K-562. No reactivity was seen with cells of T lymphocyte origin, platelets, red cells, monocytes, or acute myelocytic leukemias. Evidence is presented indicating that the determinant recognized by BA-1 is not surface immunoglobulin, HLA-DR, or receptors for C3 or Fc. We conclude that monoclonal antibody BA-1 may be useful in the study of early stages of human B lymphocyte development.