Calcium channel-blocking agents interfere with the initial increase of cytosolic calcium that follows mitogenic stimulation of T lymphocytes. In cultures of mitogen-stimulated PBMC, verapamil also blocked T cell accumulation of cytoplasmic IL-2-encoding mRNA. In sharp contrast, the addition of verapamil to PHA and PMA-stimulated PBMC augmented the mitogen-stimulated increases in nuclear transcription of IL-6-encoding mRNA, steady state levels of IL-6 encoding mRNA, and release of IL-6 bioactivity. These experiments indicated that an increased IL-6 transcriptional rate rather than stabilization of transcripts accounted for the increased cytoplasmic IL-6 mRNA levels and subsequent expression of IL-6 bioactivity. These effects were not produced by nicardipine, another potent calcium channel blocker, or EGTA. We suggest that a non-calcium-dependent, IL-6 regulatory factor, absent or inactive in verapamil-treated cultures, inhibits IL-6 gene activation in mitogen-stimulated PBMC. Failure to express this inhibitory factor would result in IL-6 gene superinduction at a transcriptional level.