The plasma cell alloantigen PC-1 was isolated from C1.18 myeloma cells by immunoprecipitation and was analyzed by polyacrylamide gel electrophoresis. It was found to consist of two similar or identical disulfide-bonded polypeptide chains, each of Mr 115,000. The mobility of PC-1 in nonequilibrium pH gradient electrophoresis was similar to that of bovine serum albumin (pI 4.9). The PC-1 antigen is therefore similar to the transferrin receptor in Mr, charge, subunit composition, disulfide bonding, and developmental regulation. Similarities can also be detected by peptide mapping with subtilisin, but not with staphylococcal V8 protease. It is suggested that the PC-1 protein and the transferrin receptor may have had a common evolutionary origin, and may have similar functions.