The biologically active form of interferon γ is a dimer composedof two noncovalently boundidentical polypeptide chains of 17 kDa each. In this study, it wasfound that dissociation of the dimerinto monomers significantly reduced the fluorescence quantum yield andthe efficiency of the intermolecularTyr to Trp radiationless energy transfer. The same process causedsignificant changes in the fluorescencedecay and in the fluorescence anisotropy decay. The kinetic andthermodynamic parameters of the dimer−monomer equilibrium were determined by fluorescence measurements atdifferent temperatures and by atheoretical mathematical model. Dissociation of the dimers intomonomers was an endothermic processand was favored by concentrations of the protein lower than 1 μM andby increasing the temperature. Itwas accompanied by formation of aggregates, a slow and partiallyreversible process leading to inactivationof the interferon. It is suggested that certain monomericconformers are competent for aggregation.