1. The temperature function of the myeloma IgG(K) IVA, Bence-Jones protein (K-type) IVA and its fragments (Fab(t), Fc'(t), VL and CL) was studied by thermal perturbation difference spectroscopy and circular dichroism. 2. The IgG and Bence-Jones protein studied were found to be capable of a fully reversible structural changes at temperatures between 25 and 35 degrees C. The changes occurring at the higher temperature are accompanied by the screening of the significant part of exposed tyrosine residues. The transition is not accompanied by an appreciable change in the main IgG secondary structure-beta-pleated sheet, according to the CD data. 3. It was found that the temperature-dependent changes of IgG occur in its Fab fragments, the changes of Bence-Jones protein occur in its variable part (VL domains). 4. The temperature changes in the interval 25-35 degrees C are explained by the flexibility of amino acid side chains composed hypervariable loops delineated the "sides" of cavity between variable domains.