(1, 3, ,5, 7â€”10)on the occurrence of protein-bound dye in the livers of rats fed the hepatic carcinogen 4-dimethylaminoazobenzene (DAB) or certain of its derivatives have directed attention to the pos sible causal role of these proteins in aminoazo dye carcinogenesis. The protein-bound dye appears in the liver a few days after the initiation of dye-feed lag and is found in the non-neoplastic portions throughout the stages of tumor formation and growth, as long as the animals are maintained on the dye. However, the protein-bound dye cannot be detected in the aminoazo dye-induced tumors1 and this lack can be interpreted as indicating a qualitative difference between the proteins of liver and those of the tumors. Over one-half of the bound dye in these livers is combined with the soluble proteins (7â€”10).The present investigation was undertaken to ascertain the electrophoretic group(s) of the soluble rat liver proteins which were combined with the dye follow ing the ingestion of aminoazo dyes that have dif ferent carcinogenic activities. In previous studies (12, 13), it was found that the soluble proteins of normal rat liver, of livers from rats fed 4-dimethyl aminoazobenzene for varying lengths of time short of neoplasia, and of liver tissue adjacent to DAB induced hepatomas had similar electrophoretic patterns. On the other hand, the soluble proteins of DAB-induced hepatomas, their metastases, and a group of other tumors showed electrophoretic patterns which were very similar to each other but different from these livers. In contrast to the non neoplastic liver tissues, all the tumors studied ex aAidedin partby research grantsfromthe Wisconsin Alumni Research Foundation, the National Cancer Institute of the National Institutes of Health, Public Health Service, and the American Cancer Society. The generousand valuable assistance rendered by Mrs. Betty F. Claus is gratefully acknowledged. t Present address, National Cancer Institute, Bethesda, Maryland.