The role of bacteria in the bioavailability of protein-bound vitamin B12 was examined in eight elderly subjects who had atrophic gastritis and in eight normal controls. On separate days and in random order, vitamin B12 absorption tests were performed using either radiolabeled crystalline or protein-bound vitamin B12. At the same time, bacterial samples were collected from the upper gastrointestinal tract. The tests and gastrointestinal aspirates were performed before and during tetracycline therapy. Crystalline vitamin B12 was absorbed to the same extent in the two study groups. Atrophic gastritis subjects absorbed significantly less protein-bound vitamin B12 than normal controls (mean +/- SEM, 0.7% +/- 0.2% vs. 1.9% +/- 0.5%, respectively). However, protein-bound vitamin B12 absorption in these subjects normalized after antibiotic therapy. These results suggest that the small amounts of vitamin B12 released from the protein binders is readily absorbed (as shown in vitro) and/or metabolized by bacteria.