Treatment of the Daudi line of human lymphoblastoid cells with concentrations of human interferons within the physiological range progressively inhibits cell proliferation over 1-4 days. Rigorous measurement of the overall rate of protein synthesis during this period, using a concentration of [3H]phenylalanine sufficient to equalize the specific radioactivity of intracellular and extracellular precursor pools, shows that protein synthesis becomes progressively inhibited as the growth inhibition develops. There is a strong correlation between inhibition of amino acid incorporation and inhibition of cell proliferation. In contrast, we find no evidence for any increase in protein degradation rate under these conditions. These results suggest that interferon treatment of susceptible cells can inhibit protein synthesis even in the absence of virus infection and that this inhibition is of a sufficient magnitude to account for the anti-proliferative effect.