Spontaneous rosetting with mouse red blood cells (MRBC) seems to be a marker for a subpopulation of B-lymphocytes. We investigated the capacity of different hematopoietic cell lines to form rosettes with MRBC. The cell lines represent different hematopoietic malignancies and the results with these lines were compared with those obtained with non-malignant EBV carrying lymphoblastoid cell lines (LCL). Among tumor cell lines only exceptional lines, 1 myeloma and 2 leukemia lines, showed rosetting capacity with MRBC. Most LCL, however, bound MRBC. These results indicate that only 1 category of established hematopoietic cell line, the LCL, has MRBC receptors and that thus a further subset of B-lymphocyte, the one represented by LCL, has the capacity for rosetting with MRBC. This MRBC rosetting of LCL is, however, not restricted to those lines with S-IgM. Tumor lines of hematopoietic origin seem, however, in general to lack MRBC receptors, even such lines that are derived from malignant lymphomas which express other B-lymphocyte characteristics.