A novel protein (BIF) which shows inhibitory activity on the binding of enterotoxigenic Escherichia coli Pb176 (ETEC with colonization factor antigen (CFA) II, which consists of coli surface-associated antigens CS1 and CS3) to gangliotetraosylceramide (asialo GM1 or GA1) was isolated from the culture supernatant fluid of Bifidobacterium longum SBT2928 (BL2928) at its stationary phase. The homogeneity of the final preparation of BIF was demonstrated by SDS-PAGE, polyacrylamide gel electrofocusing and N-terminal amino acid sequencing. The BIF was characterized as (i) a protein with an M(r) of approximately 104 kDa when chromatographed on a gel filtration column, and 52 kDa when separated on SDS-PAGE, and (ii) having an isoelectric point of 5.9. No change in size was produced by thiol reduction. These results suggest that BIF is a homodimer consisting of identical 52 kDa monomers. The purified BIF at the concentration of 25 micrograms protein ml-1 caused a 50% reduction in binding of the ETEC strain to GA1.