We assayed the plasma levels of CD8 antigen (CD8Ag) and soluble interleukin 2 receptor (sIL-2R) in 34 subjects with B cell chronic lymphocytic leukemia (B-CLL) and in 15 controls using an immunoenzymatic method. The results showed higher average levels of soluble CD8 (sCD8) and sIL-2R in the leukemic patients compared to the controls (sCD8 = 860 vs. 306 U/ml; sIL-2R = 4,131 vs. 311 U/ml). The two antigen levels were significantly higher in patients with progressive disease than in those with indolent disease, and they also correlated with Rai's stage. sIL-2R levels correlated with lymphocyte count (p less than 0.001), while there was no correlation between sCD8 levels and total number of lymphocytes. These results seem to show that the measurement of serum levels of CD8Ag and sIL-2R may be a useful tool in the prognostic evaluation of patients with B-CLL.