The transferrin receptor is encoded by a diallelic locus which had been described as the T-cell antigenic system TCA. Using a human lymphoblastoid T-cell line (HSB) which reportedly carries a transferrin receptor and which we showed to be TCA-2 positive by serology, it appeared that only a TCA-2 but not a TCA-1 specific alloantiserum had the capacity to precipitate a protein with a molecular weight of 94,000, which comigrated exactly with the Mr 94,000 protein precipitated by the rabbit anti-HSB and rabbit anti-SB antisera, as well as the transferrin receptor which was precipitated by a transferrin-antitransferrin complex. The positive sera and the latter complex were shown to cross-react in a binding assay. Conversely, a monospecific TCA-1 alloantiserum was shown to react with the Mr 94,000 protein of TCA-1-positive leukocyte suspensions. The allelomorphism of the transferrin receptor may be relevant for the eventual regulatory mechanism controlling cell proliferation, which would be essential at least for those cells that are supposed to play an immunological role like the TCA-positive cells which are described as T-cells probably involved in the process of immunosuppression. In addition, we found increasing amounts of autoantibodies with TCA-1 specificity in serum from a healthy individual (referred to as "donor" in this paper), whose brother at the same time suffered a chronic myeloid leukemia, probably associated with a retrovirus, and whose blood in contrast was shown to contain cytotoxic anti-TCB, from a diallelic locus expressed on theophylline-insensitive T-cells, and anti-B-cell activity.