By using 51Cr-labelled erythrocytes and 141Ce-labelled microspheres to correct for daily variations in faecal output, gastrointestinal microbleeding was measured in hospital patients with rheumatoid arthritis before and after administration of 2 propionic acid derivatives. These were flurbiprofen and benoxaprofen, respectively a potent and a weak prostaglandin synthetase inhibitor. Increased faecal blood loss occurred consistently with flurbiprofen and not with benoxaprofen. Our results provide indirect evidence for the cytoprotective effect of prostaglandins on the gastrointestinal mucosa in man. The quantitative correlation between faecal occult blood loss as measured by the above techniques and the Haemoccult slide test is also discussed.