Antiproliferative and antiviral activities of a type alpha human leukocytic interferon on several heteroploidic cell lines: HeLa, HEp-2, and T-10, a cell line of malignant origin (glioblastoma) were investigated, as compared to subcultures of human embryo fibroblasts. The tumor cell multiplication rate decreased proportionally to the amount of interferon in the culture medium. The highest interferon concentration used in our experiments (1,000 mu/ml) induced a decrease of the normal cell multiplication rate (human embryo fibroblasts). The same amount of interferon had a cytotoxic effect against the T-10 cells, but this phenomenon is reversible if the interferon is excluded after 24 h from the culture medium. There was no quantitative relation between the magnitude of the antiviral and of the cytotoxic effects of the type alpha human interferon on the tested cellular substrates.