The effect of human recombinant interferon alpha 2 (IFN alpha 2) on hairy cells obtained from 16 patients was evaluated. All patients promptly responded to induction of remission with 2 X 10(6) U/m2 interferon alpha 2 b, three times a week, sc. In order to achieve a more detailed insight into the mode of action of interferon in this disease, we determined the influence of IFN alpha 2 on the incorporation of radiolabeled thymidine and uridine into hairy cells. While both 3H-thymidine and 3H-uridine incorporation were unaffected by IFN alpha 2 in a 3-hour incubation period, a significant increase in uridine incorporation into hairy cells, but not CLL cells, was observed after 24 h. Cell surface marker analysis performed with monoclonal antibodies did not reveal a quantitative alteration of the immunophenotype of hairy cells in vitro. In addition, natural killer cells, assessed by monoclonal antibodies and a cytotoxicity assay against K 562 cells, were found to be decreased in 9 out of 10 patients prior to therapy. Although IFN alpha 2 could stimulate natural killer cells in vivo, we did not find a consistent correlation between the activation of these cells and the response to therapy. We conclude, therefore, that NK cells play no major role in the regression of hairy cells. Furthermore, IFN alpha 2 does not alter antigenic determinants in vitro, but leads to an enhanced incorporation of 3H-uridine into hairy cells in vitro, thus indicating a possible role for the induction of RNA synthesis in vivo.