Four Bence-Jones proteins were investigated by CD, fluorescence and analytical ultracentrifugation methods at physiological conditions (10 mM phosphate buffer, pH 7.0, 100 mM NaCl). A joint analysis of optical melting curves for proteins and their fragments were demonstrated that protein VAD has reduced stability of its constant half, which correlates with the ability of both intact protein and its constant, rather than variable part to form amyloid fibrils. Data are reported which support the viewpoint that the detected decrease in the stability is caused by abnormal interaction between a pair of constant domains C(L).