The incidence of sister chromatid exchange (SCE) and growth pattern were studied in vitro with a bromodeoxyuridine (BrdU) chromosome labelling technique in the Ph1-positive leukaemic marrow and/or blood cells from 20 patients with chronic myelocytic leukaemia (CML) at various stages of the disease. Additional chromosome abnormalities, besides the Ph1 or unusual Ph2-translocations, were found in three out of twelve cases in the chronic phase (CP), in two of four cases entering the blastic phase (BP), and in all four cases in the BP during the study. The growth pattern and SCE incidence in these cases were similar to those in cases in the BP during the study. The growth pattern and SCE incidence in these cases were similar to those in cases with a Ph1 as the sole abnormality. Ph1-positive CML cells undergoing second or more generations predominated in the marrow in the CP, whereas in the active stage of the CP and in the BP the CML marrow cells tended to go through only one generation. There were no clear correlations between the SCE incidence and the state of the CML. However, a marked increase in the SCE frequency was observed in some cases after chemotherapy, irrespective of the state of the disease. The results indicate that the type of Ph1-translocation and/or the presence of additional karyotypic abnormalities hardly affect the growth pattern and SCE incidence in the Ph1-positive CML cells, and that the in vitro BrdU-labelling methods may have a potential for detecting chemotherapeutic effects in CML patients and in the 'staging' of the clinical course of such patients.