Cryostat sections of the rat spleen were incubated with a medium containing horseradish peroxidase (HRP)-anti-HRP complexes. After this incubation HRP-anti-HRP complexes were demonstrated in the follicle centres in the splenic white pulp. The localization pattern of the HRP-anti-HRP complexes after incubation ("in-vitro trapping") was compared with the localization pattern of 125I BSA-anti-BSA complexes after intravenous injection ("in-vivo trapping"). Furthermore, the influence of endotoxin on the "in-vitro trapping" was studied. The "in-vitro trapping" was representative of the second, retaining phase of the "in-vivo trapping". Evidence is presented that endotoxin inhibits trapping of immune complexes by a direct effect on the follicular dendritic cell.