1. The temperature and pH functions of the myeloma IgG(K) conformation were studied by optical rotatory dispersion, circular dichroism, thermal perturbation difference spectroscopy, solvent perturbation difference spectroscopy, electrochemical iodination and difference adiabatic scanning microcalorimetry. 2. The IgG studied was found to be capable of a fully reversible structural change between pH 6.5 and 6.0. A transition occurring at low pH is accompanied by an increase of exposure of the chromophores to the solvent. 3. The "alkaline state" was found to be capable of a fully reversible S-like transition at temperatures between 25 and 35 degrees C. The changes occurring at the higher temperature are accompanied by the screening of 14-15 tyrosine residues and probably by a small increase in the helicity of the protein. These changes are not accompanied by an appreciable heat effect. The thermal denaturation of the "alkaline state" occurs only at 64 degrees C in the narrow temperature interval (3-4 degrees C). 4. The "acid state" is not accompanied by S-like transition at 25-35 degrees C. The thermal denaturation of the "acid state" occurs at 54 degrees C in the wide temperature interval (8-9 degrees C). 5. It was proposed that the ionisation of the invariant histidine residues situated in the "cavity" between the constant and variable domains causes the pH transition studied. The temperature changes in the interval 25-35 degrees C are explained by the alteration of the domains interposition. Similar alterations were investigated as a result of antigen-antibody reaction.