Human leucocytes from peripheral blood and tonsil were examined for the presence of the IL-3 receptor using monoclonal antibodies directed to epitopes of the alpha and beta chains of the receptor. We found that the beta chain, common to IL-3, IL-5, and GM-CSF, was either present at low levels or not detected on the majority of peripheral blood and tonsil B lymphocytes, while the alpha chain showed a distinct but restricted distribution. In peripheral blood the IL-3R alpha chain was limited to a subpopulation of peripheral B lymphocytes and a population of cells which lack lineage-specific markers. Dimly staining cells were identified as B lymphocytes as they coexpressed CD19, CD20, CD22, CD24, and HLA-DR. A brightly staining population lacks T and B lymphocyte, NK specific, and macrophage lineage markers but expresses CD9, CD45RO, CD26, and, in a proportion of cells, CD36 and CD60. This population remains unclassified. In tonsil tissue IL-3R alpha chain expression was strongest on B lymphocytes present in the T cell rich areas of tonsillar tissue. The IL-3R alpha bearing B tonsil cells included cells in both CD23 and IgD positive and negative populations. The phenotype of the IL-3R alpha positive B cells defines them as a population of B lymphocytes distinct from previously characterized cells in the lymphoid architecture. Lymphoblastoid cell lines with a corresponding phenotype were also identified.