Pteridines are heterocyclic compounds which are synthesized and released by human monocytes/macrophages following stimulation by interferon-gamma. Their concentration in various body fluids proved to be indicative for the stimulation of the cellular immune system, and determination of pteridines has become an important diagnostic tool. We show that pteridine derivatives, namely neopterin (N), 7,8-dihydroneopterin (NH2), and 5,6,7,8-tetrahydrobiopterin (BH4) increase intracellular calcium (Cai) in human monocytic cells. Significant increases of Cai are observed at 10 nmol/l NH2, at 100 nmol/l BH4 and at 1 mol/l N, i.e. at concentrations encountered in vivo. At a concentration of 1 mumol/l, Cai is increased (from a control value of 145 +/- 7 nmol/l) to 464 +/- 62 nmol/l (NH2), 340 +/- 41 nmol/l (BH4) and 344 +/- 46 nmol/l (N), respectively. The increase of Cai depends on the presence of extracellular calcium and is likely to be due to activation of a calcium channel. We show that the absence of extracellular calcium or the addition of lanthanum ions to the extracellular fluid fully reverses the pteridine-induced increase of Cai. According to these observations, pteridines may mimic the effects of other inflammatory mediators on monocytic cells and seem to be involved in the crosstalk of immunocompetent cells.