The prevalence of HBsAg was studied, by Counterelectrophoresis and Radioimmunoassay, in 232 household contacts of 65 HBsAg carriers. The overall incidence of HBsAg in household contacts was 14,65%, as compared with the generally accepted figure of about 1% in the general population in Italy. As far as the prevalence of HBsAg and abnormal liver function tests is concerned, no differences were found between contacts of carriers with Chronic Active Liver Disease and contacts of asymptomatic carriers. The prevalence of HBsAg was similar in spouses (13,5%) and in blood relatives (15,3%) of carriers. It is concluded that: 1) Household contacts of HBsAg carriers are at higher risk of acquiring HBsAg infection than the general population. 2) Such risk is not increased by the presence of Chronic Active Liver Disease in the index cases. 3) Further studies are required in order to clarify the relative roles of environmental and genetic factors as determinants of HBsAg transmission.