Nucleoside-diphosphate (NDP)-kinase can be considered to be induced by human-type interferons (HuIFNs) rapidly, since an enzyme increase was detected within 2 h of incubation of HeLa S3 cells with HuIFNs, while incubation with heterologous mouse IFNs had no such effect. The enzyme increase induced by HuIFNs reached a plateau at 6 h after treatment. Actinomycin D (0.5
) significantly blocked the enzyme increase induced by HuIFNs in the cells. A possible biological role of the enzyme in the IFN-induced biochemical events is discussed.