Bovine lactofcrrin was evaluated for its ability to enhance resistanec of rainbow trout, Oncorhynchus mykiss (Walbaum), to infection with Vibrio anguillartim. Oral administration of laclotcrrin (UKtmgkg"') to fish dail)' for 3 days prior to an intrapcriloncal challenge with V. anguillarutn resulted in inereascd survival rates, and enhaneed resistance against Streptococcits sp., although to a lesser extent. In laetoferrin-treated fish, an inercase in phagocytic and ehemilumineseent (CL) activities of proncphros cells against V. anguillarum was observed. The phagoeytic and CL aetivities of cells against Streptococcus sp. were also significantly inereascd. However, no in vitro baetcrieidal activities of lactofcrrin against 1'. anguillarum or Streptococcus sp. were observed. This suggests that the Iaetoferrin enhaneed the resistance of the rainbow trout against bacterial infection through the activation of phagocytes.