Fourteen clinical isolates of Yersinia enterocolitica serotype 0:3 and four well-documented virulent strains of serotypes 0:3, 0:8, and 0:9 were biotyped and examined for plasmid-associated autoagglutination and calcium dependency and for epithelial cell adherence. These strains were tested for the production of bacteriocin-like antagonism by using tryptone soya blood agar at room temperature and at 37°C. By using the cross-streaking method, three clinical isolates produced inhibitory substances at room temperature. These substances were active against a variety of clinical isolates and their plasmid-cured derivatives at both room temperature and 37°C. The inhibition was easier to read after incubation of the cross-streaked plate at 37°C. The inhibition patterns indicate that two of the three producer strains appear to recognize potentially virulent 0:3 strains, with or without the virulence plasmid.