The T-cell antigen receptor is composed of two variable chains (alpha and beta, termed TcR) which confer ligand specificity, and four constant chains (gamma, delta, epsilon, and zeta, collectively termed CD3) whose functions are not fully understood. To explore the role of the individual CD3 components, the human T-cell tumour line Jurkat was chemically mutagenized followed by negative selection with F101.01 (a monoclonal antibody against the TcR-CD3 complex), and cloning. Growing clones were analysed for TcR-CD3 expression by immunofluorescence. One clone, J79, was found to express greatly diminished levels of TcR-CD3. This clone produced all the TcR-CD3 components except the CD3-zeta, as demonstrated by metabolic labelling and immunoprecipitation followed by one- and two-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis. These data indicate that the CD3-zeta determines the normal intracellular fate of the TcR-CD3 complex, and that the CD3-zeta is necessary for the intracellular transport and expression at the cell surface of the TcR-CD3 complex.