The tumor antigen and cytoskeleton protein tissue polypeptide antigen (TPA) have been determined in maternal serum throughout pregnancy. TPA levels in serum from pregnant women (n = 70) were not different from those in nonpregnant women (n = 61) until pregnancy week 28, from which time levels increased until parturition. Duplex pregnancies (n = 7) had higher serum TPA concentrations than simplex pregnancies, which was significant in pregnancy weeks 28 (p < 0.014) and 32 (p = 0.05). Maternal serum TPA concentrations did not change during abortion or parturition and the mixed umbilical blood concentration of TPA equaled that in the mother’s serum just after parturition. The correlation between placenta weight and maternal serum TPA levels was significant (p = 0.025). The decline of TPA after labor indicated a t½ for serum TPA of 7.5 days.