An outbreak of hepatitis, type A, affecting 30 employees of a large metropolitan department store was investigated using traditional case findings, a food preference questionnaire, and case-control methods. In addition, the new serological tests for IgM and IgG antibody to hepatitis A virus (anti-HAV) were employed to establish a serological diagnosis of hepatitis A infection, to define the control group, and to identify the index case, an asymptomatic food handler. Twenty-five of 25 cases tested had IgM anti-HAV, whereas none of 73 controls and one of 46 food handlers were IgM anti-HAV positive. This test eliminated three suspect cases that were diagnosed as hepatitis but that had atypical epidemiologic features. Epidemiologic and serological analysis confirmed the association of illness with eating in the employees' cafeteria and eating cold sandwiches.