The purpose of the present study was to investigate the effect of interleukin-12 on apoptosis of chronic lymphatic leukemia (CLL) B cells. Apoptotic indices were determined in highly purified CD5(+) B lymphocytes isolated from peripheral blood of seven patients with histologically confirmed CLL. Interleukin-4 as a known inhibitor of apoptosis was used as control. Quantitative analysis of apoptosis was determined by cell death detection ELISA. Our findings indicate that interleukin-12 inhibits ex vivo apoptosis in a large proportion of B-CLL patients and may be closely involved in the pathogenesis of disease. Therefore, our results may help identify potential new therapeutic targets in this malignancy.