To determine the frequency of vertical transmission of hepatitis B antigen (HB5 Ag) from asymptomatic carrier mothers in Taiwan to their offspring, HB5 Ag was sought by radioimmunoassay and complement fixation. Of 158 babies born to carrier mothers, antigenemia developed in 63; 51 of these antigenemic babies had become antigen positive within the six months of life. Three inter-related factors were found to increase the risk that antigenemia would develop in the infant: a high maternal complement-fixation titer for HB5 Ag: presence of HB5 Ag in the baby's umbilical-cord blood: and antigenemia in siblings. In contrast to previous studies, these findings indicate that vertical transmission from carrier mothers frequently occurs, at least in Taiwan, and may partially explain Taiwan's high prevalence of HB5 Ag.