The antiviral and cell growth inhibition spectra of bovine and human interferons are described. The antiviral activity of type II interferon in heterologous cells far exceeded that of the corresponding type I interferon. Antiviral activity of bovine type II interferon parallels cell growth inhibition activity both in thermal inactivation kinetics as well as in the rate of synthesis. Furthermore, the spectrum of antiviral activity against heterologous cells was the same as that seen for cell growth inhibition suggesting that one molecule may mediate both effects. The therapeutic implications of type II interferon prepared in animals for use in human medicine are discussed briefly.