The linear immunogenic and antigenic structure of E. coli Gal-Gal pili from the recombinant strain HU 849 was investigated with nine synthetic peptides corresponding to regions of the pilus sequence predicted to contain hydrophilic beta-turns. Five peptides, as bovine serum albumin conjugates, were found by anti-HU 849 pilus serum and were thus designated "immunogenic epitopes." Peptides corresponding to R 25-38, R 38-50, and R 48-61 (which jointly comprise the single intramolecular disulfide loop), and R 103-116, were bound in low titer. A prominent immunogenic epitope was specified by a peptide corresponding to R 65-75. Four peptides, as thyroglobulin conjugates, elicited antisera in rabbits that bound intact HU 849 pili. These were designated "antigenic epitopes." Two prominent antigenic epitopes were localized to peptides corresponding to R 5-12 and R 93-104, whereas peptides corresponding to R 65-75 and R 119-131 represented two minor antigenic epitopes. None of the peptide antisera bound Gal-Gal pili from heterologous strains except anti-R 93-104 and anti-R 5-12. In 8 of the 10 Gal-Gal-binding pyelonephritis isolates tested, anti-R 5-12 detected a protein with an apparent molecular weight of 18,000 co-migrating with several Gal-Gal pili. Anti-R 93-104 detected a corresponding protein in 4 of 8 fecal and 7 of 12 pyelonephritis Gal-Gal-binding isolates; however, it also bound apparently unrelated proteins of higher molecular weight.