Abstract:The lysozyme activity, alternative complement activity (ACH50), respiratory burst, SOD (superoxide dismutase) activity and phagocytic activity of orange-spotted grouper Epinephelus coicoides were examined when the fish were injected intraperitoneally with sodium alginate at 10, 20, 30 mg kg−1 and ι-carrageenan at 10, 20, 30 mg kg−1, respectively after 24, 72 and 120 h. Serum ACH50 increased directly with dose after 24 and 72 h for both sodium alginate and ι-carrageenan treatments. The fish that received sodium alginate at 20 mg kg−1 after 24 and 72 h, and the fish that received ι-carrageenan after 72 and 120 h showed significantly increased respiratory burst, SOD activity and phagocytic activity, respectively. In another experiment, E. coicoides which had been injected individually with sodium alginate and ι-carrageenan at 10, 20, 30 mg kg−1, were challenged with Vibrio alginolyticus at 1.8 × 109 colony-forming units (cfu) fish−1 and then placed in seawater of 33‰. The survival of fish that received sodium alginate at 20 mg kg−1, and the fish that received ι-carrageenan at 30 mg kg−1 was significantly higher than that of fish which received saline and the control fish after 48 h as well as at the termination of the experiment (120 h after the challenge). It is therefore concluded that E. coicoides which received sodium alginate at 20 mg kg−1 or ι-carrageenan at 30 mg kg−1 increased the non-specific immune response and resistance from V. alginolyticus infection.