The chromosome constitutions of stimulated lymphocytes from 50 B-cell chronic lymphocytic leukemia patients were studied using different stimulation systems, i.e., TPA alone or associated with different cytokines. Adequate metaphases were obtained in 44 subjects (88%). Among 20 patients with abnormal karyotypes (45.5%), 7 had trisomy 12. The most frequent structural abnormality was a 14q+ resulting from translocations including one t(11;14) and two t(14;17), while deletions on the long arms of chromosomes 6 and 13 constituted a second common alteration. The most important finding in this series was the recurrence of a t(18;22) observed in two cases.