Epidemiological studies in healthy carriers of HB Ag show variable results that might depend on the geographical area, ethnic group, and socio-economic level analyzed. For that reason an study was undertaken in the Spanish population with the purpose of analysing contagiousness of healthy carriers and mechanism of spread of the infection. The incidence of HB Ag and HB Ab was determined by radioimmunoassay in 211 relatives of 76 healthy carriers; all members of the family could be studied in 51 cases. The results were compared to those of a sizeable sample of the normal population. The overall incidence of HB Ag (13.1 %), and of HB Ab (18.9 %) in the probands was significantly higher than in the normal population (0.8 %, and 9 %, respectively). The distribution of new cases of HB Ag positivity in the 51 families in which the members could be studied demonstrated a significantly higher frequency in children of a carrier mother (28.9 %) than in those of a carrier father (10.8 %). In conclusion, the contagiousness of healthy carriers for their families can not be disregarded. It is likely that the mechanism of spread of the infection depends on a multifactorial inheritance of the genetic alteration of the selective immunologic response to the HB Ag, compounded by environmental factors.