Detailed immunophenotypic analyses of immunologically classified leukemias and lymphomas showed that CD40 displays an exquisite B-lineage specificity within the human lymphopoietic system. Notably, 82% of B-lineage chronic lymphocytic leukemias (CLLs), 82% of B-lineage hairy cell leukemias (HCLs), 86% of B-lineage non-Hodgkin's lymphomas (NHLs), and 29% of B-lineage acute lymphoblastic leukemias (ALLs) were CD40+. Quantitative analyses of the correlated expression of CD40 and other B-lineage differentiation antigens on fetal lymphoid precursor cells by multiparameter two-color/three-color flow cytometry, combined with analyses of sequential antigen expression on fluorescence-activated cell fluorescence activated cell sorter (FACS) isolated immunologically distinct fetal B-cell precursor subpopulations during in vitro proliferation and differentiation, provided evidence that the acquisition of CD40 antigen in human B-cell ontogeny occurs subsequent to the expression of CD10 and CD19 antigens but before the surface expression of CD20, CD21, CD22, CD24, and surface immunoglobulin M (sIgM). Some leukemic pro-B cells from ALL patients as well as normal pro-B cell clones from fetal livers displaying germline Ig heavy chain genes were CD40+, indicating that the acquisition of CD40 antigen likely precedes the rearrangement of Ig heavy chain genes. CD40+ FACS-sorted malignant cells from B-lineage ALL as well as B-lineage NHL patients were capable of in vitro clonogenic growth, indicating the CD40 antigen is expressed on clonogenic leukemia and lymphoma cells. This hypothesis was confirmed by the ability of an anti-CD40 immunotoxin that we used as an antigen-specific cytotoxic probe to effectively kill clonogenic B-lineage ALL and NHL cells.