In 230 cases of chronic lymphocytic leukemia (CLL), marker analysis was performed with rosette techniques and a panel of xeno-antisera. A monoclonal B-cell proliferation was found in the majority of cases (94%). In most cases, the B-cells carried IgM, with or without IgD. Cytoplasmic immunoglobulin-inclusion bodies were seen in 7% of the cases of B-CLL. The number of patients with non-B/non-T-CLL was small (2%) in this series. In eight patients (4%), a proliferation of T-cells was established. These patients had a different clinical presentation and marker analysis of the lymphocytes, together with functional studies, showed that this group represented a mixture of different disease processes. Functional analysis of the B-CLL cells in 19 cases showed a poor or absent mitogen response and in nine cases the absence of the capacity to differentiate in vitro into plasma cells and/or to produce immunoglobulins.