Human interferon (HIF) preparations inhibited the propagation of Daudi cells in stationary suspension cultures, while a control preparation showed no such effect. The growth inhibition (= anticellular) activity of differently pretreated HIF preparations was determined by the reduction of 14C-labelled thymidine (14C-TDR) uptake in a microassay system. These studies demonstrated that the degree of anticellular activity is directly proportional to the antiviral activity of different HIF preparations. These preparations were obtained from peripheral leukocytes (lHIF) or diploid fibroblasts (fHIF). Together with gel chromatography results, and the sensitivity of the anticellular activity to tryptic digestion and heat inactivation, these results suggest that the anticellularly active substance is a protein which is very similar to, or identical with, interferon.