Human neutrophils deprived of divalent cations by treatment with ionophore A23187 in the presence of ethylene glycol bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA) showed superoxide release when they were preincubated with calcium and then treated with the ionophore. The release was not observed when the ionophore was added first and then calcium was added more than 5 min later. The absence of the release in this case can be ascribed to a refractoriness of the cells to stimuli, because the cells did not release superoxide on stimulation with phorbol myristate acetate (PMA). The cells pretreated with either calcium or the ionophore alone did release superoxide on addition of PMA. The refractoriness of the cells to PMA depended on the concentrations of calcium and the ionophore and on the time interval between the two treatments. Calcium could be replaced with Cd2+ but not with Mg2+, Ba2+, or Sr2+. The release of granular enzymes was observed when the depleted cells were pretreated with the ionophore and then with calcium. These observations indicate that calcium has dual effects on the superoxide release of neutrophils, i.e., it stimulates the cells and makes them refractory to stimuli, depending on the time interval after the addition of the ionophore, and it also regulates the enzyme release by a different mechanism.