Host proteins are an integral part of the HBSAg [hepatitis B surface antigen] macromolecule. The presence of host proteins may moderate the immune response to HBSAg. Asymptomatic human carriers of the hepatitis B virus may not have a generalized defect of their immune response. Rather, they may not recognize the difference between HBSAg and self because of the inclusion of host proteins in HBSAg. If this is correct, it might be expected that there would be variable responses to HBSAg, including tolerance, depending on whether the infected individual primarily recognizes the viral or some or all of the human components. The recognition of viral or human components could be under genetic control.