Calcium activation of oxygen evolution from French-press preparations of Phormidium luridum is largely reversible upon removal of added Ca(2+). Activation occurs via a first-order binding with a dissociation constant of 2.8 mM. An 8-fold increase in oxygen evolution rate observed upon Ca(2+) addition is accounted for by a 4-fold increase in the number of active photosynthetic units, and a doubling of turnover rate. While both Ca(2+) and Mg(2+) stimulate turnover, unit activation is Ca(2+) specific. Under optimal conditions, 30% of the units functioning in the intact cell can be recovered in the Ca(2+) -activated preparation. The Ca(2+) requirement of P. luridum preparations is not relieved by proton-carrying uncouplers, or by rate-saturating concentrations of the Hill acceptor, ferricyanide. Taken together with the reported stimulation by Ca(2+) of oxygen evolution in the presence of DCMU (Piccioni, R.G. and Mauzerall, D.C. (1976) Biochim. Biophys. Acta 423, 605--609) these observations strongly suggest a site of Ca(2+) action within Photosystem II. The pronounced specificity of the Ca(2+) requirement appears in preparations of other cyanobacteria (Anabaena flos-aquae and Anacystis nidulans) but not in the eucaryote Chlorella vulgaris. While milder cell-disruption methods bring about some Ca(2+) dependence in P. luridum, French-press treatment is required for maximal expression of Ca(2+) -specific effects. French-press breakage causes a release of endogenous Ca(2+) from cells, supporting the view that added Ca(2+) restores oxygen evolution by satisfying a physiological requirement for the cation.