Water-soluble proteins from Aeromonas sobria, a causative agent of bacterial hemorrhagic septicemia of fishes, were separated into six fractions by gel chromatography on Sephadex G-100. Injections of fraction II (67 kDa) provided the highest protection of carps against the disease. Injections of proteins contained in fraction II caused stronger effects on certain biochemical parameters in the fish liver (fatty acids of phospholipids and cathepsin B and D activities) in comparison to infections of the live culture.