This report describes an instance of possible homologous viral interference in the isolation of IPN virus from carrier fish. Brook trout (Salvelinus fontinalis), suspected of being carriers of IPN virus, were examined for the presence of the virus by inoculating monolayer cultures of RTG-2 cells with different dilutions of tissue homogenates of the suspected fish. Cultures of RTG-2 cells infected with concentrated tissue homogenates exhibited little, if any, cytopathic effect (CPE) and yielded little infectious IPN virus at 72 h after infection. However, cell cultures infected with a relatively high dilution of tissue homogenate exhibited extensive CPE and resulted in high yields of infectious virus.