Peripheral blood cells of twenty-six patients with B cell chronic lymphocytic leukemia (B-CLL) were characterized for their surface membrane and cytoplasmic marker profiles using flow cytometry and fluorescence microscopy. According to surface membrane marker analysis three distinct immunophenotypic subgroups of B-CLL were identified: group I (SIg+, MR+, CD5+, B Ag+, T Ag-; 19 cases), group II (SIg+, MR+, CD5+, B Ag+, TAg+; 3 cases), group III (SIg-, MR+, CD5+, B Ag+, T Ag-; 4 cases). Cells from all patients were positive for the CD19 antigen and at least one of other B cell antigens. Cells from all patients expressed also CD5 and HLA-DR antigens and formed mouse rosettes (MR). Great heterogeneity was found in the membrane and cytoplasmic marking by anti-CD22 MoAb. In four of 23 patients tested, CD22 antigen was expressed in the cytoplasm of CLL cells while it was absent on surface membrane of these cells. This finding was discussed from the point of certain cell heterogeneity in the followed B-CLL cases. Cytoplasmic immunoglobulin (CyIg) detection showed to be very important especially in group III of followed B-CLL cases with undetectable surface immunoglobulins (SIg). Cytoplasmic antigens and immunoglobulin determinations are useful in phenotyping every B-CLL patient, as well as in the immunological study of different maturation stages of B lymphocytes.