The mode and nature of the binding of chlorpromazine (CPZ), a psychotropic drug, with myoglobin, a monomeric muscle protein, were studied spectrofluorometrically and the results compared with those from the binding of CPZ to hemoglobin, a tetrameric allosteric protein from red blood cells (RBC). CPZ interacted with myoglobin in a non-cooperative mode, with a binding constant of 8.4 × 103 M-1 in 0.145 M NaCl, pH 6.8, whereas in the case of hemoglobin this interaction was found to be positively cooperative with a binding constant of 4.2 × 103M-1. The interaction of CPZ with myoglobin was not influenced by the NaCl molarity of the solution, whereas CPZ interaction with hemoglobin significantly decreased with increasing NaCl molarity, indicating that CPZ-hemoglobin binding is mostly electrostatic in nature, whereas that of the CPZ-myoglobin complex is of a non-electrostatic type. Thermodynamic analysis revealed that binding of CPZ to hemoglobin was exothermic (ΔH° = - 2.65 kcal/mol), whereas binding to myoglobin was endothermic (ΔH° = + 1.39 kcal/mol) with a high entropic contribution (ΔS° = + 23 cal/degree/mol), suggesting that CPZ binding to myoglobin is hydrophobic in nature. Such contrasting binding features of this drug have been discussed in the light of a typical subunit interaction property present and absent in hemoglobin and myoglobin, respectively. © 1994.