The quality of peptic ulcer healing does not only mean complete epithelial restitution of the mucosal surface but also adequate repair of the underlying connective tissue. To obtain more information about the metabolism of extracellular matrix proteins in gastric mucosa and submucosa, we investigated biopsy specimens from six patients with antral peptic ulcers and six normal controls by staining of collagen types I, III, and IV with an immunofluorescence technique. In normal mucosa we found a certain amount of collagen types I and III in equal distribution and almost no collagen type IV. In contrast, there was a remarkable increase of collagen types I and III in peptic ulcers predominantly located at the ulcer edges. These results are compatible with the view that extracellular matrix proteins play some part in the ulcer healing process.