Two flagellar antigens, intact flagella and monomeric flagellin from Salmonella adelaide were labelled with 125I by the Chloramine-T method. They were injected into the hind foot-pads of rats, and the localization in the draining lymph nodes was studied by scintillation counting and autoradiography. Injected rats belonged to one of four groups: normal, unimmunized adult rats (NI); rats that had been given an unrelated antigen 6 weeks previously (HI); rats that had been given a priming dose of the same antigen (though unlabelled) 6 weeks previously (AI); and rats that had been given passive antibody by intraperitoneal injection (PI). Follicular localization was more rapid in AI and PI groups than in NI or HI rats. With flagellin, but not with flagella, the final follicular concentration reached was also greatly increased. No differences were observed between NI and HI rats, or between AI and PI rats. In primary lymphoid follicles, the antigen was distributed throughout the follicle in a diffuse network, presumably of macrophage fibrils. In secondary follicles, the antigen localized in a crescentic cap occupying the superficial aspect of the follicle. The study stressed the importance of antibody acting as an opsonin in determining details of antigen localization. [ABSTRACT FROM AUTHOR]