Despite its world-wide distribution, the pathogenic serogroup O: 5, 27 of Yersinia enterocolitica (Y. e.) has scarcely been considered in the Federal Republic of Germany. In the 1980 to 1986 period, a total of 74 strains from patients was identified at the National Reference Centre for Salmonella, Hamburg. The clinical manifestations of the cases was not different from those associated with Y. e. O: 3 and O: 9 infections. Six out of 8 patients from whom serum samples were available, developed moderate agglutination titres (1:80-1:1280). Only 8 of the 74 strains were negative in in vitro virulence tests. A list of biochemical tests is given which permit a differentiation of the pathogenic serogroup O: 5, 27 from the apathogenic Y. e. O: 5 which is widely distributed in the environment. Serological studies revealed different H antigens in both serogroups. The O antigen factor 27, and a subfactor O: 5(2) which is typical of Y. e. serogroup O: 5, are further differential markers; the production of both factor sera is described. Besides serogroups O: 3 and O: 9, Y. e. O: 5, 27 must be considered as an aetiological agent of human yersiniosis in the Federal Republic of Germany.