Allicin was fed at 0 (= control), 0.5 and 1.0 mL of Allimed liquid 100 g(-1) of feed for 14 days to rainbow trout, Oncorhynchus mykiss (Walbaum), fingerlings before infection with Aeromonas hydrophila with a resultant reduction in mortalities from 80% in the controls to 8% [relative percentage survival (RPS) = 90%] and 0% (RPS = 100%) among the treated fish. Allicin was strongly antibacterial compared to the control, with a minimum inhibitory concentration (MIC) of >400 microL mL(-1) of Allimed liquid. Use of allicin led to a lower number of white blood cells (132.0 +/- 0.4 x 10(3)) compared to 175.0 +/- 0.1 x 10(3) in the controls, but elicited increased phagocytic activity, i.e. a phagocytic value of 39.2% compared to 13.6% in the controls, and serum lysozyme activity, which showed significant (P > 0.05) differences compared to the control at 15 and 30 min after the first reading at 0 min of incubation.