A 78 kDa protein was induced in cat peripheral blood mononuclear cells after in vivo administration of recombinant human interferon-alpha hybrid (rHuIFN-alpha B/D). This protein was antigenically related to the IFN-induced human (78 kDa) and mouse Mx proteins. Quantitative immunoblot analysis indicated that the induction of the cat Mx protein was dose-dependent. There was a dissociation in time between plasma levels of IFN which were transient, and levels of cat Mx protein which remained elevated at least five days after dosing. Our results provide evidence that a human IFN-alpha hybrid may be active in cats. They also indicate that the Mx protein is a sensitive, quantitative, and stable marker to follow IFN activity in vivo in cats.