The occurrence and serum concentrations of adenocarcinoma-associated antigen (ACAA) were studied during the fetal period of life, in newborns and in their mothers. The mean concentrations were significantly higher in fetal, newborn and maternal sera when compared with the mean concentration of ACAA in healthy, nonpregnant adults. Thus, ACAA appears to show fetospecific features as is known for other oncofetal proteins. ACAA should be recognized not only as a potential tumor marker, but also as a normal protein constituent of human serum.