Twenty-one teenagers who were exposed to the common source during an outbreak of gastroenteritis were tested for seroconversion to the Norwalk virus. Serum pairs were collected within 72 hours of exposure and four weeks later. Each of the 11 subjects who developed symptoms and five persons who remained well demonstrated a whole serum antibody response, while five non-ill subjects failed to seroconvert. The absence of detectable antibody in the acute-phase serum specimens was not statistically associated with subsequent seroconversion or susceptibility to illness. These findings underscore the view that immunity to this agent is not determined by serum antibody and support the concept that susceptibility may be determined by Norwalk-specific intestinal receptor sites.