In order to obtain more information on the pattern of CD11c-positivity in otherwise typical B-cell chronic lymphocytic leukemia (CLL), we analyzed immunological and clinico-pathological features of 99 such patients studied with two different monoclonal antibodies (MoAbs). Fifty-two out of 70 (74.2%) patients stained with IOM-11C MoAb and 3 out of 29 (10.3%) patients stained with Leu-M5 MoAb had more than 30% CD11c positive cells (P < 0.0002). The two groups were similar with regard to the expression of B-cell CLL-related antigens (CD5, CD20, CD23), as well as clinico-pathological features (i.e. Binet clinical stage and pattern of bone marrow involvement), thus suggesting that differences in CD11c expression were due to different reactivity patterns of the MoAbs utilized. In our experience, the use of different reagents may affect immunophenotyping results, thus providing conflicting data at times.