Anti-isotypic reagents against the human T cell receptor (TcR) were made by immunizing rabbits with peptides which corresponded to sites within the constant region of the alpha- and beta-chains. These antibodies were shown to immunoprecipitate a heterodimer of 80,000 to 90,000 m.w. that could be reduced to chains of 44,000 to 50,000 and 37,000 to 40,000 m.w. In addition, an anti-peptide serum against CD3 delta-chain was made. The anti-alpha peptide serum reacted with all human TcR (from phytohemagglutinin-stimulated lymphocytes, cytotoxic T cell clones, and the T cell leukemias: HPB-ALL, Jurkat, JA3, and JM), and the anti-beta peptide serum reacted with only human TcR of the C beta 2 isotype (from a cytotoxic T cell clone which had a C beta 2 transcript, HPB-ALL, and a proportion of phytohemagglutinin-stimulated lymphocytes, but not with Jurkat, JA3, and JM). A comparison of the detergents NP-40 and digitonin revealed that digitonin was more efficient at keeping the TcR/CD3 complex intact, but was less efficient at solubilizing the total amount of TcR or the total amount of CD3. With these reagents and the use of digitonin, it was shown that all of the alpha, beta, and CD3 moieties on the surface of a T cell leukemia HPB-ALL occur as a bound TcR/CD3 complex. The proportion of C beta 1 to C beta 2 isotype expressed on the surface of phytohemagglutinin-stimulated peripheral blood lymphocytes was 0.8, indicating approximately equal use of the two beta-chain isotypes.