The plasma protein-thyroid hormone complex was investigated by (1) the binding (paper and starch gel electrophoresis) of I131-L-thyroxine and I131-L-tri-iodothyronine by plasma proteins, (2) the passage of T4 and T3 from such protein linkages across a dialysis membrane and (3) the in vitro incorporation of T4 and T3 by human erythrocytes. Results indicate (1) electrophoretic heterogeneity of the "albumin" and "inter-alpha globulin" serum proteins which preferentially bind thyroxine, (2) a major effect of Tris (hydroxymethylaminomethane) buffer on both overall thyroxine binding capacity of serum and the relative T4 binding by the "pre-albumin" and "inter-alpha globulin," (3)a significant effect of pH on binding, (4) a striking inhibitory effect of Veronal buffer on T4 binding by serum albumin.