A total of 12 hematopoietic cell lines was tested for in vitro sensitivity to interferon (IF). The cell lines were 4 T-cell ALL lines (TALL-1, MOLT-4, HPB-ALL, HSB-2), 1 ATL line (MT-1), 2 null-cell ALL lines (NALL-1, KOPN-8), 1 pre-B-cell ALL line (NALM-18), 1 B-cell line (BALL-1), 1 myeloma line (Oda), 1 Burkitt lymphoma line (JBL-3) and 1 APL line (HL-60). At a concentration of 1,000 U/ml, all 4 T-cell ALL lines, NALM-18, NALL-1 and Oda showed high or moderate sensitivity to the growth inhibition effect of IF. BALL-1 showed slight sensitvity and its in vitro growth was considerably inhibited at a higher concentration of IF (5,000 U/ml). However, MT-1, JBL-3, KOPN-8 and HL-60 were resistant to IF and their in vitro growth was not inhibited at all even at the concentration of 1,000 U/ml. MT-1 cells, although resistant to IF in suspension culture, were sensitive in agar culture. Colony formation was markedly inhibited at a concentration of 1,000 U/ml. The mechanism of growth inhibition by IF and the implications of the present findings are discussed.