Abstract In the present study the B-cell mitogens from Epstein-Barr virus (EBV) and lipopolysaccharide-B from E. coli (LPS) were used to stimulate the blood cells of 27 patients with chronic lymphocytic leukemia (CLL) to undergo mitosis. In 14 CLL cases EBV was used as the only mitotic stimulant and in 5 cases both EBV and LPS. In 8 cases neither EBV nor LPS caused mitotic cells to appear. In 9 cases the cells were characterized by abnormal clones, including 5 cases with +12 (including 1 case with a possible Philadelphia chromosome), 2 cases with 14q+ and 1 case with an i(17q). No correlation was found between the types of surface membrane immunoglobulins and the chromosome abnormalities in the leukemic lymphocytes. The effects of EBV on normal lymphocytes were also investigated and found to be nonspecific (e.g., tetraploidy), although in every case EBV caused a definite increase in the mitotic index. The results indicate that 1) EBV and LPS stimulate leukemic cells of CLL to undergo mitosis, 2) about 50% of the patients have clonal abnormalities in the leukemic cells, and 3) CLL is associated with karyotypic changes seen in lymphoma (14q+), although the most common anomaly in our material was a +12.