Rabbits were immunized with the enterobacterial common antigen (ECA)-immunogenic strain Escherichia coli F470. ECA-specific antiserum was obtained by absorbing the resulting antisera with the genetically closely related ECA-negative strain E. coli F1283. These two strains also served as positive and negative controls in the localization study of ECA in Yersinia enterocolitica strain 75, smooth and rough forms (Ye75S and Ye75R), by the indirect immunoferritin technique. Cells of Ye75S grown at 22 degrees C showed no labeling with ferritin after treatment with the ECA-specific antiserum and subsequent ferritin-conjugated goat anti-rabbit antibodies. If the cells were grown at 40 degrees C, however, most of the cells showed weak ferritin labeling. At this higher growth temperature, the lipopolysaccharide of this strain contains less O-specific chains (6-deoxy-L-altrose), as was shown in a previous study. The rough mutant Ye75R, which lacks O-specific chains completely, showed denser labeling with ferritin. These results indicate that ECA on the cell surface of Ye75S is covered by O-specific chains of the lipopolysaccharide if grown at 22 degrees C and is therefore not accessible to ECA antibodies. It becomes accessible, however, when O-chains are lacking (R mutants) or when they are reduced in size or amount (growth at 40 degrees C).