A galactose-specific adhesin was isolated from the fimbriae of an enteroaggregative Escherichia coli (EAEC) strain. The adhesin was found to be a high molecular weight aggregate of the 18-kDa monomer. The dimeric (36 kDa) and tetrameric (76 kDa) forms appeared in sodium dodecyl sulphate polyacrylamide gel electrophoresis when a higher concentration of the adhesin was used. The IgG(AD) (IgG against adhesin) obtained from the immune sera raised in rabbits against purified adhesin could detect all three forms of the adhesin even from the crude fimbrial preparation. The IgG(AD) failed to recognize the adhesin in the presence of galactose, thereby suggesting the antibody-binding site and the sugar-binding site on the adhesin might be same or overlapping. Furthermore, the IgG(AD) could localize the adhesin exclusively on the fimbriae as observed in immunogold electron microscopy. The aggregative adherence of the bacteria to HEp-2 cells was reduced to 70% in the presence of the IgG(AD). A glycoprotein (34 kDa) present in the membrane fraction of HEp-2 cells interacted with the purified adhesin in a galactose-specific manner. The IgG(AD) could recognize the adhesin from the crude fimbrial preparation of 9 out of 10 clinical isolates of EAEC strains but failed to identify any protein from the crude fimbrial preparation of Salmonella typhimurium (fim +ve as well as fim -ve strain), Vibrio cholerae (WO7) or Escherichia coli DH5 alpha.