Evidence from developing countries and volunteer studies indicates that immunity to Campylobacter jejuni and Campylobacter coli may be acquired, but the antigenic basis for this protection is poorly defined. We have purified to homogeneity four proteins with molecular weights of 28,000 (PEB1), 29,000 (PEB2), 30,000 (PEB3), and 31,000 (PEB4) from epidemic C. jejuni strain 81-176 using acid extraction and sequential ion-exchange, hydrophobic interaction, and gel filtration chromatography. The relative amino acid compositions of these four proteins are similar. NH2-terminal sequence analysis indicates that all four proteins are different, although the first 35 amino acids of PEB2 and PEB3 are 51.4% homologous. Isoelectric focusing showed that all four are basic proteins with pI of 8.5 for PEB1 protein and greater than 9.3 for the others. Use of the purified proteins as antigens in an IgG enzyme-linked immunosorbent assay (ELISA) found that seroconversion to the PEB1 or PEB3 proteins occurred in 15 of 19 patients with sporadic C. jejuni or C. coli infection. In comparison, only two, six, and 14 of these patients seroconverted to PEB2, PEB4, or the acid extract antigen. In an ELISA with whole bacterial cells as antigens, antiserum to the acid-extracted antigens showed broad recognition of C. jejuni, C. coli, C. fetus, C. lari, and Helicobacter pylori. Antiserum to PEB1 recognized all 35 C. jejuni and all 15 C. coli strains but none of the isolates of the other three bacterial species. The PEB1 and PEB3 proteins appear to be candidate antigens for both a Campylobacter vaccine and for serological assays for the pathogen.