Purified photochemical reaction centers from Rhodopseudomonas sphaeroides R-26 were reduced with Na2S2O4 so as to block their photochemical electron-transfer reaction. The magnetic field induced an increase in the emission yield. Our results support the hypothesis that under these conditions, charge recombination in the singlet radical pair composed of the oxidized primary donor and reduced primary acceptor predominantly generates the excited singlet state of the reaction cnter bacteriochlorophyll. The maximum relative fluorescence change and the value of the magnetic field at which half-saturation of the effect is achieved (B1/2) at room temperature are 5.5% and 75 G, respectively. For the whole cells of Rps. sphaeroides R-26 these parameters are 1.2% and 120G. The relative fluorescence change at 600 G, deltaF/F(600), and B1/2 are studied as functions of temperature. The temperature dependencies of deltaF/F(600) for reaction centers and whole cells of Rps. sphaeroides R-26 are qualitatively the same, with the maximum effect (8% for reaction centers) occurring at 230 K. However, the B1/2 curves for the two preparations are different.