We carried out in vitro B cell colony assays, with phytohemagglutinin-P (PHA-P) and IL-4, on B cells from one intermediate lymphocytic lymphoma (ILL) patient and four chronic lymphocytic leukemia (CLL) patients. Peripheral blood B cells from the ILL patient responded to PHA and IL-4, they proliferated, and differentiated into cells with plasmacytoid cell morphology. They lost the CD19 surface antigen after 10 day co-culture with PHA and IL-4. The bone marrow of this ILL patient contained atypical plasma cells with multiple nuclei. Peripheral blood B cells from the four CLL patients responded to PHA, but IL-4 did not increase PHA-induced B cell colony formation in these cells. The CLL cells did not differentiate into plasma cells, and they were clearly different from the ILL cells in morphology, as shown by scanning electron microscope examination. Since this study was performed on cells from only one ILL patient, further examination of cells from many patients might be necessary to confirm the difference between ILL and B cell-type CLL.