In nine patients with CLL treated with chlorambucil followed by alpha-2b-interferon (alpha 2b-IFN), T, B and natural killer (NK) cells and NK activity were studied before entering the study, after chlorambucil treatment, and after administration of alpha 2b-IFN. When considered as a whole, basal NK activity was lower in CLL patients than in controls (21.0% +/- 10.9 versus 40.2% +/- 17.4, p less than 0.001); however, when considered individually, four out of nine patients had normal NK activity at diagnosis. Chlorambucil did not increase global NK activity (21.7% +/- 7.1), whereas alpha 2b-IFN did so (44.3% +/- 19.1). After alpha 2b-IFN only one of seven patients studied had low NK activity. Previously increased absolute counts of CD2+, CD4+, CD8+, CD16+, CD57+ lymphocytes were reversed with chlorambucil treatment to normal levels, while after this therapy CD11b+ and CD19+ cells decreased without reaching normal values. During alpha 2b-IFN therapy, an increase up to normal levels in the percentage of CD16+ (2.7% +/- 3.4 versus 7.7% +/- 6.5, p = 0.04) and CD57+ (3.0% +/- 3.0 versus 8.1% +/- 6.2, p = 0.020) lymphocytes was observed whereas the absolute number of CD19+ B-cells further decreased (5.2 x 10(9)/l +/- 2.5 vs 3.8 x 10(9)/l +/- 2.3), albeit not significantly.