Malignant B cells from hairy cell leukemia (HCL) patients are unable to proliferate when stimulated with standard B cell mitogens. Using chromatographically purified B cell growth factor (BCGF), HCL can be stimulated to proliferate as assessed by incorporation of tritiated thymidine [3HTdR] into DNA. Proliferation was found to be time dependent, with no detectable 3H-TdR incorporation in up to three days of culture, and significant stimulation evident at days 6 and 10. The presence of 10% BCGF in culture was an absolute requirement for HCL proliferation; however, this BCGF-induced DNA synthesis could be further augmented by the addition of anti-immunoglobulin heavy chain antibodies. BCGF-induced proliferation was abrogated in six of six patients by addition of 1,000 U/mL of recombinant alpha 2-interferon (IFN) at day 0, although 1,000 U/mL of recombinant gamma-IFN had no inhibitory effect in five of six patients studied. Specific cellular receptors for type I IFN were demonstrated in HCL by inhibition of binding of 125I-alpha 2-IFN by a 40-fold excess of unlabeled alpha 2 or beta IFN with no inhibition by unlabeled gamma-IFN. These data demonstrate that malignant HCL lymphoblasts express specific type I IFN receptors and that type I, but not type II IFN, can inhibit growth factor-induced DNA synthesis by hairy cells in vitro. They further suggest a direct antiproliferative mechanism of action for IFN in HCL and predict equivalent clinical activity by either alpha or beta, but not gamma IFN in this malignancy.