Abstract. An insoluble polysaccharide from the cell wall of Saccharomyces cerevisiae, called M-Glucan, has been shown to enhance the non-specific disease resistance of Atlantic salmon, Salmo salar L., when injected intraperitoneally. M-Glucan consists only of glucose units which presumably are linked through β-1,3 and β-1,6 linkages. Enhanced resistance was demonstrated against Yersinia ruckeri, the causal agent of enteric redmouth disease, against Vibrio anguillarum, the causal agent of classical vibriosis and against Vibrio salmonicida, which causes cold water vibriosis or ‘Hitra-disease’ in salmon. At a dose of 2mg M-Glucan per fish (20g mean weight), maximal resistance developed in the fish 3 weeks after injection. Injection of different glucan doses and challenge one week later with Vibrio anguillarum, showed that 50-200μg glucan per fish resulted in the highest level of resistance. The level of resistance in Atlantic salmon obtained with M-Glucan was strikingly higher than that obtained with another glucan which was prepared from Saccharomyces cerevisiae by a different procedure.