The possibility of intrauterine transmission of the Australia antigen (HAA)-associated hepatitis virus was examined in three pregnancies. Each of the three trimesters of pregnancy were represented by an instance of maternal HAA-positive hepatitis occurring during that period. A variety of immunological techniques were applied to test for HAA, HAA-antibody, and nonspecific serum and tissue inhibitors in fetal conceptus organ tissues and sera from maternal, fetal, and placental sources. Our methods failed to detect HAA in umbilical cord and fetal blood as well as in fetal tissue studies. Tissue and umbilical cord blood inhibitory substances could not be demonstrated. A total of 12 instances are now available in which umbilical cord blood has been HAA negative in the presence of recent or concomitant HAA-positive hepatitis or persistent antigenemia. This evidence now suggests that transplacental transmission of HAA from mother to fetus, if it occurs, is rare.