Serologic evidence of exposure to Australia (Au) antigen prior to conception, as indicated by the presence of Au antigen or Au antibody during pregnancy, was not more frequent in 58 mothers who delivered infants with Down's syndrome than in 58 mothers who delivered normal babies. Australia antibody was detected in 13.8% (eight of 58) of the serum samples of mothers delivering infants with Down's syndrome and in 25.8% (14 of 58) of the matched control mothers. Probable placental transfer of antibody was observed in ten of 11 cases in which there was detectable maternal Au antibody. While no association was found for serum hepatitis and Down's syndrome, the possible association of maternal infectious hepatitis and Down's syndrome could not be assessed as there are as yet no reliable tests for infectious hepatitis virus or antibody.