The process of photoactivation has been studied in dark grown cells of Chlamydomonas reinhardtii. A mutant, FUD 39, lacking the Cl--concentrating 23-kDa psbP protein of photosystem II was found to have a decreased capability to perform photoactivation. The yield of the process never reached wild type level, and contrary to the wild type, it was highly dependent on the intensity of the activating light, with a very narrow optimum around 1 microE m-2 s-1. The different behavior in the mutant can be explained by a requirement for a longer dark period, between the two photoacts, during the photoactivation. This is proposed to reflect the decreased Cl- affinity in the mutant. Photoactivation in the mutant was also found to be very sensitive to competing photoinhibitory processes. The inhibition was located to the donor side of photosystem II and affected the photoactivation capability before electron transfer from Tyrz was inhibited. We propose an extended model for photoactivation in which an intermediate that is sensitive to photoinhibition is formed if Cl- is not functionally bound to the manganese cluster.