The protein-blotting technique was used to determine the antigens of Toxoplasma gondii that were recognized by IgG and IgM antibodies in sera of congenitally infected newborns and their mothers. Patterns of IgG and IgM blots with sera from newborns revealed antigen-antibody reactions (bands) that were not present in the respective blots obtained with sera from their mothers. This was true for 50% of 24 congenitally infected newborns. In contrast, such a difference was noted in only one (5%) of 21 newborns who were not congenitally infected but whose mothers had serological evidence of acute infection with T. gondii acquired during gestation. Our results suggest that the protein-blotting method or an adaptation may be valuable for study of the immune response of the mother, fetus, and newborn to various antigens of infectious organisms and for diagnosis of congenital infections in the newborn.