Hepatitis B antigen (HB Ag) was detected by complement fixation (CF) in serum samples of 7.5% of 1,106 pregnant Chinese women tested in Taipei, Taiwan. HG Ag persisted in all but one of 42 women followed for 1 to 18 months (average, nine months) after delivery, and 27 of the 43 infants (63%) born to those women became antigen-positive. Persistance of the antigen was more common than transient or intermittent antigenemia. Twelve had antigenemia when first tested, while 15 later developed antigenemia, usually during the first six months of life. Only one infant developed antibody to HG Ag (anti-HB Ag), and this occurred after transient antigenemia. The HB Ag was found in two of 32 (6%) fathers, and in 18 of 27 (67%) older siblings. The antigen was more common among siblings of antigen-positive than among those of antigen-negative infants. These findings demonstrate that in Taiwan, infants born to mothers who are asymptomatic carriers of HB Ag commonly become infected by heaptitis B (HB) virus. Exposure of infants near the time of birth may be important maintaining the high, chronic HB Ag carrier rate in Taiwan.