The light minus dark difference spectrum and the kinetics of the indicator pigment C-550 have been measured at room temperature in isolate, envelope-free chloroplasts in the presence of 3-(3' ,4'-dichlorophenyl)-1,1-dimethylurea (DCMU). The C-550 spectrum indicates a band shift with peaks at 540 and 550 nm and has an isobestic point at 545 nm. On the assumption of 400 chlorophyll molecules per electron transfer chain the differentaial extinction coefficient delta epsilon (540-550) is calculated to be approximately 5 mM-1 . CM-1. The kinetics of the C-550 absorbance change, occurring upin the onset of continuous illumination, are shown to be biphasic and strictly correlated with the kinetics of the complementary area measured from the fluorescence induction curve under identical cinditions and with those of the absorbance increase at 320 nm due to photoreduction of Q. The lighted-induced change in these three parameters can be described as a function of the variable fluorescence yield change occurring under the same conditions. Such functions are non-linear and reveal a heterogeneous dependence of the variable fluorescence yield on the fraction of closed System II reaction centers. It is concluded that for every molecule of the primary electron acceptor Q of Photosystem II that is photochemically reduced there corresponds an equivalent change in the absorbance of the indicator pigment C-550 and in the size of the complementary area. Ths, C-550 and area are two valid parameters for monitoring the primary photochemical activity of System II at the room temperature.