Chlorpromazine is found to be taken up by the erythrocytes in a dissociable binding similar to that of the plasma protein binding so that an equilibrium is attained between these binding sites. The plasma/cell ratio is on an average 2 but is subject to large interindividual variations. This ratio is only moderately influenced by the presence of other drugs and anticoagulants in the blood. The consequences of the cell binding of drugs for the analytical result when using plasma or whole blood and the evaluation of blood concentrations of their relation to the therapeutic effect are discussed.