A striking clinical response of hairy cell leukemia (HCL) to alpha-interferon (alpha-IFN) has recently been reported. To explain its efficacy on HCL, we investigated the ultrastructural changes caused by alpha-IFN in hairy cells. Mononuclear cells (MNCs) were separated from the peripheral blood of eight HCL patients in the leukemic phase. Aliquots of 1 X 10(6) MNCs in 2.5 ml of RPMI 1640 medium were cultured with a recombinant DNA-derived alpha-2 interferon (RD alpha 2-IFN) for 1, 3, and 7 days. The concentrations of IFN added were 10, 100, 1,000, and 10,000 IU/ml culture medium. Electron microscopic examination revealed that RD alpha 2-IFN induced the formation of tubuloreticular structures in cultured hairy cells and increased the frequency of hairy cells having annulate lamellae (ALs), probable precursors of the nuclear pore. Hairy cells from various patients showed a variability in the extent of tubuloreticular structure (TRS) formation. Excellent clinical responses, as judged by hematologic parameters, were observed among patients whose leukemic cells were susceptible to RD alpha 2-IFN with respect to TRS formation in vitro. from these findings, we propose that the induction of TRS formation, in combination with the increase of ALs, may be a morphologic expression of a direct antitumor action of IFN on malignant cells.