Sera from seventy patients suffering for HBsAg positive hepatitis, twenty-five HBsAg blood-donors, and eighty-four healthy subjects with no history of hepatitis, negative for all markers of HBV infection, were tested for circulating IgM/HBsAg complexes by a radioimmunoassay. IgM/HBsAg complexes, present in acute hepatitis patients at admission, progressively decreased and could not be demonstrated in sera of the convalescent phase. No complexes were found in sera from subjects negative for HBV markers. The results obtained from sera from HBsAg positive blood donors are compared with HBeAg/anti-HBe system. The absence of IgM/HBsAg complexes seems to be a sign of good prognosis in HBV infection.