The effect of a single intravenous dose of calcium channel blocking agent verapamil (VER) on immune parameters in humans remains uncertain. In this study the effects of VER on lymphocyte subpopulations, interleukin 1 (IL-1) and 2 (IL-2) production in vitro, autologous mixed lymphocyte reaction (AMLR), IL-2 receptor expression (Tac positive cells) and natural killer (NK) cell activity were assessed. The investigations were undertaken on 12 hospitalized men, aged 19-23 years, with small abdominal complaints. None of them had active duodenal ulcer disease, while in the remaining no signs of organic disease were found. VER was given intravenously in a dose of 0.15 mg/kg and examinations were performed on peripheral blood mononuclear cells (PBL) drawn before and 30 and 150 min following VER injection. The single VER dose induced a significant, but transient increase in T, T helper, T suppressor lymphocytes and monocytes, and decrease in IL-1 and IL-2 generation as well as diminished Tac antigen expression. These data provide evidence that calcium channel blockers may transiently disturb immunoregulation.