Antisera were prepared against Morganella morganii (Proteus morganii) type strains from the scheme described by Rauss et al. (Acta Microbiol. Acad. Sci. Hung. 22:315--321, 1975) and Raus and Vörös (Acta Microbiol. Acad. Sci. Hung. 6:233--248, 1959; Acta Microbiol. Acad. Sci. Hung. 14:195--198, 1967). The specificities of the somatic (O) antigens were studied using the passive hemagglutination and slide agglutination techniques. Previously unreported interstrain relations were observed, and O groups were provisionally defined on the basis of related strains. Of 143 isolates, collected mostly from two hospitals, 96% could be placed in one or another of the O groups. Forty-eight percent belonged to O group 1 and most of these were of two serotypes O1ab, 2 (20%) and O1ad, 2 (12%).