Peroral jejunal biopsies were performed in seven normal volunteer subjects prior to, 48 hours after and two weeks after the administration of the Hawaii agent of viral gastroenteritis. Light and electron microscopic examination revealed an intact mucosa with blunted villi, shortened and distorted microvilli, swollen mitochondria and intercellular edema. These histologic changes were seen only in acutely ill volunteer subjects and were absent two weeks after illness in three of four who were previously ill. This reversible lesion was similar to, but not identical with, that previously described in viral gastroenteritis induced by the Norwalk agent. Serum antibody increases in response to the Hawaii agent as measured by immune electron microscopy were present in three of four ill volunteer subjects and in none of three who remained well.