This report shows that chicken type I and type II interferons (IFNs), like their mammalian counterparts, act synergistically such that a mixture of the two has much greater activity than that expected from the separate contribution of each type. The degree of antiviral synergy was measured by virus plaque reduction and cytopathic effect (CPE) inhibition in both primary and secondary chicken embryo cell cultures. Mixtures of the two Ch-IFNs produced antiviral effects 3-10 times greater than that expected from strict additivity of each IFN acting alone. At high concentrations of IFN mixtures there was a qualitative shift to an exponential IFN action does-response curve that revealed synergistic effects greater than 100-fold. Synergy resulted even with mixtures containing less than 1 U/ml of either type of Ch-IFN. The antiviral effects developed more rapidly with mixtures than when type I or II Ch-IFN was used alone. Mixtures of the two types of Ch-IFN synergistically potentiated nitric oxide secretion in cells of the HD11 chicken macrophage line. Molecular mechanisms are cited that may account for synergy between type I and type II IFNs, and speculation is offered on the epidemiologic and therapeutic implications of synergy in vivo.