In studies of the mechanism of neomycin sulfate-induced malabsorption, this drug was administered orally and by tube to the jejunum and ileum in 11 subjects. Studies of fat, calcium, sodium, potassium, and nitrogen excretion were carried out in both urine and feces. Reduction in the steatorrhea was produced by the progressively more distal placement of neomycin. Fecal sodium, potassium, and nitrogen levels were less affected by the site of antibiotic delivery, but the fecal calcium content was greatest with jejunal instillation. Divergence between changes in fecal fat and calcium levels was observed consistently. Urinary calcium, sodium, and potassium levels were frequently greater when neomycin was delivered into the jejunum or ileum than when it was given orally. The administration of bile salts, sodium bicarbonate, and pancreatic enzymes did not consistently affect the steatorrhea.