Lymphocytes from 14 patients with chronic B-cell leukaemia (B-CLL) and one with chronic T-cell leukaemia (T-CLL) were studied by the MAC (Morphology, Antibodies, Chromosomes) method, which allows simultaneous analysis of the morphology, immunologic phenotype and karyotype of the same mitotic cell. Use of the MAC-method in present studies has yielded new information about the cytogenetics of CLL. Although most of the interphase cells from patients with B-CLL were positive for B-cell markers, many of the mitotic cells turned out to be T cells, supporting the notion that the cells studied by conventional chromosome analysis are often non-neoplastic T cells. In some B-CLL cases with normal karyotype in the conventional chromosome study, however, most of the mitotic cells were B cells, indicating that neoplastic B cells may also have a normal karyotype. The patient with T-CLL had normal karyotype even though most of the mitoses were T cells. The chromosome abnormalities found were restricted to cells with light chain clonality. Our results show that clonal chromosome abnormalities do occur in neoplastic B cells of patients with B-CLL.