The CD3 epsilon and zeta chains of the TCR have been shown to possess independent signaling capabilities. Studies with chimeric molecules containing the cytoplasmic domains of either zeta or epsilon have suggested that these two structurally distinct members of the TCR-CD3 complex are able to function autonomously and have redundant features in the context of TCR-signal transduction in mature T cells. Expression of a chimeric human IL-2-receptor-zeta-chain molecule in the CD4+8+ T-cell line, DPK, has enabled us to directly analyze responses initiated by the zeta-chain-signaling module alone within the context of immature T-cell differentiation. In this paper, we show that antibody crosslinking of the chimeric zeta chain delivers only a limited activation signal as measured by Ca[2+] flux, induction of low-level CD5 expression, and minimal differentiation as assessed by loss of cell-surface CD8 expression. TCR-induced activation through antibody crosslinking of the endogenous CD3 epsilon receptor in the absence of costimulation was also relatively inefficient in initiating activation and differentiation. However, co-crosslinking of the CD4 coreceptor with CD3 resulted in a synergistic response, where as there was little effect of co-crosslinking of CD4 and the zeta-chain chimera. Striking differences were also observed in the substrate pattern of tyrosine phosphorylation, as well as lymphokine secretion following triggering through the intact TCR versus the zeta chain alone. These results indicate that although the zeta-chain may possess some signaling capacities similar to that of the intact TCR, it appears to have limited function as an autonomous subunit in initiating CD4+8+ T-cell differentiation.