A series of experiments were conducted to determine the effects of rainbow trout serum (RTS), taken from rainbow trout (Oncorhynchus mykiss) that have never experienced infectious pancreatic necrosis disease, on infectious pancreatic necrosis virus (IPNV) in vitro and in vivo. In vitro, 109 IPNV isolates belonging to various subtypes and having various degrees of virulence were passed in the presence and absence of 1% RTS on rainbow trout gonad (RTG-2) cell line. Moreover, brook trout (Salvelinus fontinalis) were challenged with four virulent Buhl subtype isolates passed 11 times in the presence and absence of 1% FITS. Avirulent isolates of IPNV were inhibited from replication on RTG-2 cells even after a single passage in the presence of 1% RTS, whereas isolates having a high degree of virulence were consistently resistant to RTS. Moreover, in vivo the level of inhibition was related with virulence of the virus (P < 0.0001). Serum inhibition of RTS on IPNV growth in vivo and in vitro is not dependent on the virus serotype. The effect of RTS presence on virus virulence in virus propagation was variable even in the same serotype.