The capacity of serum albumin to bind bromsulphthalein (b.s.p.) has been studied using the technique of gel filtration. Data from each of four groups of subjects (pregnant women, newborn infants, women taking an oral contraceptive, and male controls) show a virtually hyperbolic relationship between the molecule per molecule binding of b.s.p. and the concentration of albumin. There was also a significant difference between the binding capacity of albumin from the newborn and that from the other three groups, amongst which there were also significant differences.