The mechanism of inhibition of the photoactivity of spinach chloroplasts by lipolytic enzymes was investigated. Potato galactolipase and venom phospholipase A_2 caused a rapid inhibition of the photoactivity. These inhibitions were, however, largely prevented by addition of bovine serum albumin to the mixtures. Evidence also indicated that the fatty acids and lyso-lipids produced by the enzymes can be removed from the chloroplast membranes by bovine serum albumin. These exhibit that the enzymic inhibitions occur principally due to the lipid digestion products. Thus, a comparison between the digested lipids and enzymes in the inhibitory reactions was then performed. The inhibition by the digested lipids was less specific but the enzymic inhibition was specific for either photosyste m I or II reaction. Also, the digested lipids induced a large spectral change of the chloroplasts but the enzymes gave a little. From the findings it seems that lipolytic enzymes attack specific site of the membranes and inhibit the electron flow near the site, while the added digested lipids are freely adsorbed all over the exposed suface of the chloroplast membranes and bring about a less specific inhibition.