Two flagellar antigens, intact flagella and monomeric flagellin from Salmonella adelaide were labelled with 125 I 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).