Cell surface structures related to differentiation, activation and “homing” were identified on the leukemic cell clone in blood of 64 patients with a monoclonal B-cell lymphoproliferative disorder. Patients were selected with regard to clinical signs and symptoms of the disease. 39 patients had progressive chronic lymphocytic leukemia of B-cell type (B-CLL): 16 with lymph node enlargement and 14 with progressive lymphocytosis as the most prominent symptom, respectively. 1 patient had an isolated splenomegaly and 8 had symptoms from enlarged lymph nodes, lymphocytosis and/or splenomegaly. 25 patients had an isolated monoclonal B-cell lymphocytosis in blood and bone-marrow but no other signs or symptoms of the disease. The lymphocytosis in these patients was considered to be of “undetermined significance” and the term B-cell lymphocytosis of undetermined significance (B-MLUS) was used. Patients with a prominent lymphadenopathy and/or splenomegaly had CD22+ leukemic cells while in patients with a progressive lymphocytosis the B-cell clone expressed Leu-8. Thus, CD22 might be related to the homing capacity of B lymphocytes for lymphnodes and spleen, while Leu-8 might define a circulating B-cell subset. In B-MLUS about 50% of the monoclonal B cells co-expressed Leu8 which is consistent with a more differentiated phenotype compared to B-CLL with progressive lymphocytosis. The CD22 expression was mostly low in B-MLUS although a few patients showed high values. The expression of receptors for growth factors (CD23, CD25, CD71) was higher in B-CLL compared to B-MLUS patients (p < 0.001), which is consistent with a difference in lymphocyte activation stage and/or response to growth factors.