Foundamental Courses of Aquatic Products Science. Studies on antibacterial protein of bullfrog serum infected by Aeromonas hydrophila. To investigate the antibacterial protein in bullfrog (Rana catesbeiana) serum，bullfrogs were injected intraperitoneally with Aeromonas hydrophila suspension with a concentration of 1×107 cells·mL-1. The antibacterial activities of sera from infected bullfrogs in the period of 7 d post-injection were higher than those of control bullfrogs injected i.p. with sterile physiological saline. The serum antibacterial activity of infected bullfrogs peaked at 3 d post-injection and had a significant difference (P <0.05) to the control counterpart at 3 d and 7 d post-injection. Another group of bullfrogs were injected i.p. with Aeromonas hydrophila, and the sera were collected 3 d post-injection. The sera were separated by Sephadex G-25 gel filtration. The results of OD280 showed that there were 2 main peaks of the fractions collected. The antibacterial activity of serum to Aeromonas hydrophila was determined and the results showed that the first peak contained the main antibacterial protein. The antibacterial range of the antibacterial fractions was determined by using 10 strains as indicator, among them, Escherichia coli, Bacillus subtillis, Pseudomonas aeruginosa were inhibited. Result of SDS-PAGE assay showed high-molecular-weight of the antibacterial protein. The results indicated that bullfrog would produce multi-antibacterial-component soon after being infected by Aeromonas hydrophilia; the major antibacterial protein had high-molecular-weights and inhibited some Gram-positive and Gram-negative bacteria. 24 Refs.In Chinese. Key words:Rana catesbeiana； Aeromonas hydrophila；antibacterial protein；antibacterial activity ZOU Wen-zheng(Fisheries College of Jimei University, Xiamen 361021,China), ZHANG Jun-jie, YAN Qing-pi, JI Rong-xing //J Fish China,2007,31(1).62-67