Milk is commonly recommended as therapy for patients with peptic ulcer. The purpose of this study was to examine the effect of various forms of milk and 0.15 M NaCl (control) on gastric-acid secretion in five patients with duodenal ulcer during a period of remission and in five normal subjects. A significant (P less than 0.05) increase in acid secretion in both groups was produced by 240 ml of whole, low-fat, and nonfat milk. The acid secretory responses to milk were equivalent to approximately 20% to 35% of maximal betazole - or pentagastrin-stimulated acid output. Gastric-acid secretion produced a significant (P less than 0.05) increase in acid secretion in the patients with duodenal ulcer but not in the normal subjects. Because milk contains both protein and calcium, and each are stimulants of gastric-acid secretion, there is reason to question its frequent ingestion by patients with peptic ulcer.