Antisera specific forSpirillumthermolabile antigens (TLA) of 29Spirillumstrains were obtained by adsorption of antisera prepared against unheated cells with homologous heat-treated cells. Each antiserum was tested against all of theSpirillumstrains by cross-agglutination. Cross-agglutinations occurring at serum dilutions of 1:160 and above were considered indicative of a sufficient degree of antigenic relationship to permit serological grouping, and the antigenic relationships within such groups were supported by adsorption tests. The TLA antisera were used successfully to identify unknownSpirillumisolates. The high specificity ofSpirillumTLA antisera allowed serological distinction of most of the species ofSpirillumfrom one another and from members of related genera.