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1. [3H]ouabain binding to human erythrocyte membranes is a time- and temperature-dependent process. The association of ouabain to the membrane-bound receptor follows second-order kinetics, while the dissociation is a monomolecular reaction. An association rate constant of 4-6 x 10(4) M-1 sec-1 and a dissociation rate constant of 1-4 x 10(-4) sec-1 were measured at 37 degrees C. The dissociation constant calculated from these data agrees with that determined from equilibrium binding experiments. There is only one type of ouabain binding sites with high affinity for the drug as reflected by the low dissociation constant of 0-28 x 10(-8) M. 2. The dissociation constants of the ouabain-receptor complexes from human erythrocyte and cardiac membranes are identical. 3. The maximal number of membrane-bound ouabain binding sites was measured from equilibrium binding experiments as 288 +/- 28 per single erythrocyte. Thus one receptor site corresponds to less than 1 mum2 of the membrane, provided the receptors are diffusely distributed on the surface of the membrane. 4. Neither the maximal number of ouabain receptors nor the affinity for the drug changes with the age or sex of the blood donor. 5. A maximal transport capacity for sodium of 5-6 m-equiv/hr.1. is calculated from the number of receptor sites per erythrocyte and from the turn-over number of the (Na+ + K+)-ATPase.