Ca 2+ channels of the L-type were characterised in intact human peripheral blood lymphocytes using a radioligand binding technique and the dihydropyridine-type Ca 2+ channel antagonist [ 3H](+)-PN 200-110 (isopropyl-4-(2,1,3-benzoxadiazol-4-yl)1,4-dihydro-5-methoxycarbonyl-2,6-dimethyl-3-pyridine carboxylate) as a ligand. [ 3H](+)-PN 200-110 binding to human peripheral blood lymphocytes was time-, temperature-, concentration-dependent and of high affinity. The dissociation constant ( K d) value was 0.4 ± 0.02 nM and the maximum binding capacity ( B max) was 33.5 ± 1.6 fmol/10 6 cells. Pharmacological analysis of [ 3H](+)-PN 200-110 binding to human peripheral blood lymphocytes was consistent with the labelling of a Ca 2+ channel of the L-type. In fact, dihydropyridine derivatives were the most potent competitors of [ 3H](+)-PN 200-110 binding, whereas phenylalkylamine and benzothiazepine compounds or non-selective Ca 2+ channel modulators were weak or ineffective displacers. These findings are the first observation that human peripheral blood lymphocytes express Ca 2+ channels of the L-type. The possibility that Ca 2+ channel antagonists may interfere with immune system function is discussed.