The T-cell receptor for antigen (Ti) was purified from the human tumor cell line HPB-ALL. Amino-terminal sequence analysis of an acid-cleaved peptide of the Ti alpha chain showed that it is highly homologous to a putative murine alpha chain recently described. Amino-terminal sequence analysis of the Ti beta chain revealed that it shares 50 percent homology with the Ti beta chain amino acid sequences from two other human T-cell tumors. Nucleotide sequence analysis of a complementary DNA clone encoding the Ti beta chain from the HPB-MLT cell line showed that this chain represents a second human constant region gene segment and suggested that it arises from direct joining of the variable and joining gene segments without any intervening D region sequences.