We describe a specific and quantitative novel assay for biologically active human type I interferon (IFN), the MxR assay. It is based on a Vero cell line containing multiple copies of a hybrid gene consisting of the murine Mx promoter, which is responsive to type I IFN, linked to the human growth hormone (hGH) transcription unit. Exposure of this cell line to IFN-alpha or -beta for 12-48 hours results in the production of hGH that is measured by a commercially available radio-immune assay. The response to IFN-alpha is dose-dependent between 3 and 1000 units/ml. There is no response to TNF, IL-1 and a number of other cytokines and growth factors, and only a negligible response to IFN-gamma.