Blood groups according to ABO and Rh systems were tested in 137 ulcer patients as well as the secretory state. The titre of blood group ABH antigens was determined in the same patients, secreted in saliva and gastric juice. The results are compared with those obtained from the examination of a control group of 30 clinically healthy subjects. X-sex chromatin in the cells of biopsy mucosa was examined in 40 ulcer patients, taken from the ulcer area after gastroscopy. As a control, X-sex chromatin was studied in smears from exfoliated oral mucosa in the same subjects. The data confirm the presence of genetic predisposition to ulcer disease development in subjects from blood group O (alpha, beta) and non-secretory type. Low titres of blood group antigens were found in the secretory patients. Differences in X-sex chromating characteristics were observed between bucal mucosa and the mucosa from the ulcer area.