A stable light-induced EPR signal is reported in photosystem II particles and in chloroplasts at 5 K. Characteristic spectral features indicate that the signal arises from dipole-dipole interactions of a radical pair triplet state. From its dependence on potential, its relationship to the spin-polarized triplet state, and the redox state of the pheophytin acceptor (Ph) and because it is present in Tris-washed chloroplasts but not in untreated chloroplasts, we conclude that the signal is formed when the reaction center is in the state D(+)P(680)Ph(-) (P(680) is the primary chlorophyll donor and D(+) is an oxidized donor to P(680)). The low-temperature photochemical sequence is thought to occur as follows. (i) Donation from D to the P(680) (+)Ph(-) state occurs at liquid helium temperature in low quantum yield; this reaction is reversible at temperatures above 5 K. (ii) In normal chloroplasts, donation occurs to the D(+)P(680)Ph(-) state, but this does not occur in Tris-washed chloroplasts or in the photosystem II particles at 77 K or lower. (iii) Illumination, at 200 K, of photosystem particles or Tris-washed chloroplasts results in donation to the D(+)P(680)Ph(-) state from another donor. From experiments in the absence of redox mediators and the temperatures dependence of the splitting of the signal, it is suggested that the state D(+)P(680)Ph(-) itself may be the origin of the radical pair triplet signal. The signal has been simulated by assuming the presence of at least two distinct radical pairs that differ slightly in the distance separating the radicals of the pairs. The distance between the radicals of the pair is calculated to be 6-7 A.