A sensitive radioimmunoassay technique was used to detect hepatitis B e antigen (HBeAg). A strong correlation was found between HBeAg positivity of the serum of hepatitis B surface antigen (HBsAg) carrier women in Taiwan and the subsequent development of surface antigenemia in their babies. All babies who became chronic HBsAg carriers were born to HBeAg positive women, maternal HBeAg positivity being a better prior indication of chronic antigenemia developing in the baby than the HBsAg titer in the mother's serum.