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The fractional binding of 14C-salicylic acid and 14C-pentobarbital was studied as a function of age in plasma of the pig, dog, goat and human. The pig exhibited an unusual degree of hypoalbuminemia and low fractional binding of salicylic acid at birth. Albumin levels and the percent binding of salicylic acid were not significantly lower in newborn plasma than in adult plasma of the other species studied. The pig was also exceptional in that the high fractional binding of pentobarbital observed at low Iigand concentrations in plasma of the newborn was not observed in the other species studied. Depletion of albumin in newborn pig plasma did not appreciably affect the binding of pentobarbital. Thus it is suggested that the plasma of the fetal and newborn pig may contain a nonalbuminoid protein, such as a fetal globulin, which is capable of binding pentobarbital but does not have a high avidity for salicylic acid. It apparently does not exist at birth in the plasmas of the other species studied, as depletion of albumin markedly reduced the fractional binding of pentobarbital.