The serum samples collected from 1279 healthy subjects were studied by radio-immunologic methods before they were given polyvalent immunoglobulin for hepatitis A prophylaxis. They departed all together for the endemic area at the same time. 1220 remained healthy, and serum samples (120 th day) were studied once again. Hepatitis A occurred in 59 cases. For 30 studied of them, the proof had been given at the initial phase, either by the discovery of the virus in the stool, or by detection of anti-HAV IgM. Among the subjects having anti-HAV antibodies, these, whose titer was around hundred, were better protected; the risk seemed more important for a smaller or higher anti-HAV titer. Among the subjects who came back with hepatitis A, the markers of HB had been more frequently observed in the first sample. When their anti-HAV antibodies titer is under 20, the markers of HB are significantly fewer often associated. This might mean either that these is a congenital susceptibility towards the two viruses or that prior infection by virus HB enhances the risk of hepatitis A.