The capacity of immune-complex (IC) trapping was examined using purified horse radish peroxidase (HRP)-anti-HRP (PAP) on frozen sections of mouse spleen in vitro. We investigated the trapping mechanisms by applying the IC with or without fresh mouse serum added on the spleen sections of naive as well as immunized mice. When the PAP was applied alone, it mainly located on the macrophages in red pulp. In the splenic white pulp of immunized mice, PAP was trapped on follicular dendritic cells (FDC) in a small area of the germinal center whereas it scarcely bound to the splenic white pulp of non-immunized mice. An antibody against mouse Fc receptor (2.4G2) blocked the trapping but antibodies against mouse complement receptor (8C12, Mac-1 and 7G6) did not. When the PAP was applied mixed with fresh mouse serum, it bound on FDC in the primary follicles in the spleen of non-immunized mice. The density and area of IC trapping increased in the spleen of immunized mice. IC trapping in the presence of fresh mouse serum was blocked by the antibodies 8C12 and 7G6 but not by 2.4G2 or Mac-1.