Treatment of Daudi B lymphoblastoid cells with interferon (IFN)-alpha or -beta has been reported (Yap, W. H., Teo, T. S., and Tan, Y. H. (1986) Science 234, 355-358) to cause a transient increase in the level of diacylglycerol, which is the endogenous activator of protein kinase C (PK-C). To assess the role for PK-C in the induction of 2-5A synthetase mRNA and activity by IFN-alpha in Daudi cells, we have examined the effects of PK-C inhibitors and activators. We have found that the PK-C inhibitor, 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), strongly inhibits the induction of 2-5A synthetase mRNA and activity by recombinant IFN-alpha A (rIFN-alpha A) and that this inhibition appears to be at a post-transcriptional level. The inhibition by H7 could not be attributed to a generalized decrease in macromolecular synthesis or to inhibition of the binding or internalization of rIFN-alpha A. Moreover, pretreatment of Daudi cells for 24 h with phorbol esters to down-regulate or desensitize PK-C substantially inhibited the subsequent induction of 2-5A synthetase mRNA and activity by rIFN-alpha A. These data suggest that PK-C activity is required for the induction of 2-5A synthetase by rIFN-alpha A. However, phorbol esters, which are potent PK-C activators, did not induce 2-5A synthetase. Taken together, our data indicate that a functional PK-C is required for 2-5A synthetase induction by rIFN-alpha A at a post-transcriptional level in Daudi cells but that activation of PK-C is not sufficient for induction of this enzyme.