In 11 of 23 volunteers the Norwalk virus-like particle was visualized by immune electron microscopy in at least one stool specimen obtained during the acute phase of experimentally induced nonbacterial gastroenteritis. Examination of multiple stool specimens obtained during the course of illness in these 11 volunteers revealed maximal concentration of Norwalk virus-like particle at the onset of illness and shortly thereafter; in no case was the Norwalk particle visualized in stools obtained before the onset of illness. This finding further suggests that the Norwalk particle was the etiological agent of the Norwalk gastroenteritis outbreak. The limit of reliability of our immune electron microscopy assay system for particle counting was examined.