Aims: Characterization of the aggregation-promoting factor (APF) of the human intestinal isolate Lactobacillus crispatus M247 and its homologous nonaggregating mutant Mu5. Methods and Results: Western blot analysis revealed that the supernatant of both M247 and Mu5 contains a 28-kDa protein which cross reacts with the antiserum produced against the APF of Lact. gasseri 4B2. The apf genes of M247 and Mu5 strains were identical and were shown to be 672 nucleotides in length and encoding a protein of 223 amino acids with a predicted molecular weight of 24.0 kDa. Conclusion: Our results shows that the lost of aggregation in Mu5 is not related to a defect in secretion of the APF protein or a mutation in the apf gene. Significance and Impact of the Study: These results suggest that the mutation in Mu5 may be contained in another molecule involved in aggregation such as a possible receptor for APF.