Salmonella organisms of an unknown serotype are inoculated in the center of a motility agar plate, and paper discs impregnated with antiflagellar antisera are placed in the periphery of the plate. The plate is incubated at room temperature overnight. During this time, the bacteria spread in a widening circle toward the discs, while the antiserum from each disc, in turn, diffuses centrifugally. When the motile organisms encounter an antiserum reacting with their flagella, they are immobilized. A semicircular line of immobilization is noted around the reactive antiserum disc. Eleven different Salmonella isolates were typed in duplicate by a standard method and by the immuno-immobilization method. Results obtained by the two methods were essentially identical. Simultaneously, single phases were isolated from the zone between the immobilization line and its antiserum disc. Isolates from this region were of the phase not immobilized by the antiserum disc. The dried discs, prepared in tris(hydroxymethyl)aminomethane buffer and stored at 4 C, were stable for at least 5 months. The method can be used for the study of relatedness of surface antigens of motile, growing bacteria, thus circumventing the need for solubilization of these antigens. The results obtained can be interpreted in a similar fashion to the "identity"-"nonidentity" lines of the Ochterlony double-diffusion technique for soluble antigens.