The CD3 6 gene encodes multiple transcripts regulated by transcriptional and post-transcriptional mechanisms* CD3 is a multi-subunit complex of proteins noncovalently associated with the T cell receptor (TcR) for antigen. Considerable evidence indicates a role for CD3 molecules in the transduction of activation signals in Tcells. The murine CD3 6 gene encodes a 0.7-kb transcript present in mature T cells. Here we report the characterization of several additional CD3 6 transcripts; two nuclear transcripts, 4-4.5 kb in size, and two predominately cytoplasmic transcripts of 1.5 kb and 2.5 kb. BothT lymphomacell lines and normal thymocytes express the 1.5-kb and 2.5-kb CD3 6 transcripts.These cytoplasmic transcripts have long 3'-untranslated sequences which extend beyond the polyadenylation site of the predominant 0.7-kb transcript. The protein synthesis inhibitor cycloheximide (CHX) increases the expression of all three cytoplasmic CD3 6 transcripts, indicating that their level of expression may be regulated by a labile inhibitor protein(s). The C H X elicited increase in CD3 6 mRNA appears t o result from post-transcriptional events since the rate of CD3 6 gene transcription remains constant. In contrast to CHX, the calcium ionophore A23187 increases the rate of C D 3 6 gene transcription and, like CHX, also increases the level of cellular CD3 6 mRNA. The immunosuppressive agent cyclosporin A inhibits A23187-mediated stimulation of transcription, but has no effect on the CHX-mediated induction of CD3 6 mRNA. We conclude that both transcriptional and post-transcriptional mechanisms can regulate the amount of all three cytoplasmic CD3 6 transcripts.