Immune electron microscopy, which can detect hepatitis A antigen and antibody (anti-HA), was used to study a foodhandler-associated outbreak of hepatitis among 136 naval recruits. In stool specimens collected during the acute phase of illness, 27-nm viruslike hapatitis A antigen particles were shown, but only in patients with icteric hepatitis. Detection was possible in stools collected as early as 10 days before peak serum aminotransferase activity and up to the time of peak enzyme activity, but not thereafter. The immunologic similarity of these viruslike particles to those found in acute phase stools of volunteers inoculated with the MS-1 strain of hepatitis A virus was determined, and an increase in anti-HA was shown between acute and convalescent serums from 25 of the recruits. These data support the view that the MS-1 strain of hepatitis A virus is serologically related to naturally acquired type A hepatitis.