The light scattering by sericin-A solution applied to the criterion for its preparing conditions: time required for extraction, ionic strength and pH of extracting medium, were determined in M/20 phosphate buffer pH 7.5, and we obtained the following results : 1) Molecular weights of sericin-A, cooling water soluble fraction, when extraction from raw cocoon is carried in water solution at 100℃ were not altered by heating periods, M=118,000 for water extraction and 50,000 for extraction by water adjusted with dilute NaOH to pH 9.0. Although molecular weights are unchangeable with heating period the protein interactions, B, refered by Scatchard were decreased owing to increasing of heating periods. Since the quantity of interaction is proportional to charge and axial ratio of protein molecule, the decreasing of interaction implies that the decreasing of charge and axial of sericin-A have been raised with heating in water solution. 2) We concluded that conversion of sericin-B, most similar fraction to native sericin existing in silk worm gland and having strong gel forming ability, to A was caused by the loss of net charge and globulizing of B molecule with heat energy. On the contrary it is noteworthy that the depolymerizations of A molecule were not observed in heating period by light scattering. 3) In extraction of alkali solution pH 9.0, extracted B are easily subjected to translation to A and simultaneously degradated to small molecule having half molecular weight. 4) The ions of neutral salt in extracting medium, if salt solution is used for extraction, provide the decreasing of dissociation and avoid the decomposition on protein charged groups, but in experimental result we obtained at present that this effects not appeared and consequently all values by light scattering are same to one of water extraction.