Lack of expression of a cell surface protein can occur by means of transcriptional and/or post-transcriptional mechanisms. Expression of the CD8A gene has been shown to be regulated by post-transcriptional mechanisms when 1) CD4(-)CD8(lo) thymocytes are blocked from differentiating into CD4(+)CD8(+) cells by TCR crosslinking and 2) upon activation of mature CD8(+) T cells. We demonstrate in this paper that there is also post-transcriptional regulation of CD8A expression in a CD4(+)CD8(-) T-cell line Jurkat. On the basis of northern blotting, mRNA for CD8A was not seen to be present in the Jurkat cells, but the gene was observed to be transcriptionally active in nuclear run-on analysis. In addition, we provide evidence that post-transcriptional mechanisms also contribute to the regulation of CD8A expression in mature CD4(+)CD8(-) T cells, challenging the assumption that the regulation is due solely to transcriptional mechanisms.