Heparinized cord blood from a newborn child and peripheral venous blood from three other individuals have been cultured in vitro to investigate whether the frequency of sister-chromatid exchange (SCE) can be affected by the presence of human Namalwa lymphoblastoid interferon (Namalwa IFN). The results demonstrated that both purified and crude Namalwa IFN failed to show higher yields of SCE than normal controls (P greater than 0.05). No Namalwa IFN-dependent variation was found in the frequencies of SCE as well. Our findings seem not to support the view that Namalwa IFN, produced by Epstein-Barr virus (EBV)-transformed lymphoblastoid cells, is a carcinogen or mutagen.