Light-induced fluorescence yield changes and low-temperature changes were measured in spinach and Chlamydomonas reinhardi chloroplasts before and after treatment with pancreatic lipase. Lipase treatment destroyed the C-550 absorption band and eliminated the fluorescence of variable yield. In spinach chloroplasts the invariant fluorescence yield, after lipase treatment, was low whereas in Chlamydomonas chloroplasts it was high. Lipase treatment modified the cytochrome b559 in spinach chloroplasts so that it was no longer ascorbate reducible but was dithionite reducible. In Chlamydomonas chloroplasts some of the cytochrome b559 was destroyed by lipase treatment. After lipase treatment, cytochrome b559 could be reduced by Photosystem I activity if ascorbate was present as an electron donor and benzyl viologen present as an electron acceptor for Photosystem I. Lipase also eliminated the EPR Signal II.