In order to study the function of the intestinal epithelium in the terminal ileum, the Schilling test was performed in 10 patients with Yersinia-triggered reactive arthritis, in 10 patients who had recovered from Yersinia enteritis without complications, and in five patients with rheumatoid arthritis treated with non-steroidal anti-inflammatory agents. The Schilling test indicates absorption of vitamin B12 in the terminal ileum, i.e. the area affected by Yersinia and inflamed in patients with reactive arthritis. The findings obtained demonstrate increased uptake through the epithelium in this area of the intestine in patients with Yersinia-triggered reactive arthritis. There are two possible explanations. First, Yersinia infection may have a long-term effect on the gut mucosa. Secondly, some individuals may, at the level of the terminal ileum, show enhanced absorption of vitamin B12 and/or other substances such as microbes or their components, resulting in increased susceptibility to certain infections.