It was found that intraperitoneal administration of scleroglucan (a β-1,3-glucan from Sclerotium glucanicam) greatly enhances the phagocytic activity of carp head kidney phagocytes, and that the glucan also strongly activates the complement system of carp in vitro. To elucidate the mechanism underlying this enhanced phagocytosis, head kidney leukocytes obtained from glucan-injected and non-injected fish were separated by Percoll discontinuous density gradient centrifugations into lymphocyte (L), macrophage (M) and neutrophil (N) fractions, and each fraction was subjected to a rosette formation test using EAC (sheep erythrocytes sensitized with carp antibody and complement) as indicator cells. M and N fractions from glucan-injected fish showed high rosette formation compared with those from non-injected fish, and the rosetting was inhibited by treatment of EAC with anti-carp C3 or by depletion of Mg 2 + . This indicated that carp phagocytes increased the expression of C3-receptors analogous to mammalian CR3, which recognize iC3b in the presence of Mg 2 + , after administration of scleroglucan. On flow cytometric analysis utilizing a mAb (mouse) against the C3-receptors of carp neutrophils, it became evident that in glucan-injected fish, almost all neutrophils expressed C3-receptors.