Activation of spontaneously dividing T cell hybridomas induces interleukin-2 (IL-2) production, a cell cycle block, and programmed cell death. T cell hybridomas that express the T cell antigen receptor (TCR) ζ homodimer (ζ2), but not the TCR ζη heterodimer, were studied. The ζη- cells produced little or no inositol phosphates (IP) when stimulated with antigen. In most cases the hydrolysis of phosphoinositides was also impaired after stimulation with antibody to CD3, although one ζη- cell produced normal concentrations of IP. The ζη- cells slowed their growth and secreted IL-2 in response to both stimuli. However, the ζη- cells did not the after activation with antigen. Since activated thymocytes also undergo programmed cell death, these results may have important implications for the role of the ζη TCR in negative selection.