Peripheral blood lymphocytes (PBL) from 107 untreated patients with chronic lymphocytic leukemia (CLL) were analyzed for the presence of surface immunoglobulin (Ig) and the ability to form rosettes with sheep erythrocytes (SRBC). Four groups were identified based on the cell surface markers: (1) 81 patients' PBL expressed primarily IgM kappa or IgM lambda, 4 further patients' PBL expressed IgM with equal percentages of kappa and lambda surface markers; (2) 13 patients had equal percentages of PBL expressing lg and SRBC receptors; (3) 6 patients' PBL primarily formed rosettes with SRBCs, and (4) in 3 patients and the majority of cells had no detectable markers (null cells). Lymphocytes from all patients within each group were tested for their ability to respond to phytohemagglutinin (PHA) and pokeweed mitogen (PWM). The maximum response in PHA-stimulated normal cell cultures appeared at 2--3 days; for PWM-stimulated cultures, maximal response was at 3--5 days. CLL cultures from all patients in each of the four groups required 5--7 days to develop a maximal PHA response. The response of CLL lymphocytes in all groups to PWM stimulation was similar to normal lymphocytes. Thus, the abnormal PHA response of CLL lymphocytes was independent of the presence or pattern of cell surface markers.