ABSTRACT: While the phagocytosis of Renibacterium salmoninarum by rainbow trout macrophages in vitro occurred in the absence of serum factors, opsonized bacteria once phagocytosed survived for a longer period than bacteria not exposed to serum. Immune sera favoured survival to a greater extent than non-immune sera. Although heat-inactivated immune sera reduced the extent of bacterial killing, bacterial persistence was enhanced to such an extent when both complement and antibodies were present, that opsonized bacteria grew faster within macrophages than extracellularly. The effect of serum was observed following 16 h, but not 3 h, of prior exposure of the bacteria to serum, suggesting that opsonization per se was not the basis for improved survival, but that the serum induced an adaptative change in the bacterium.