Summary THe cryoprotective properties of bovine serum albumin (BSA) were investigated with thylakoid membranes isolated from spinach leaves ( Spinacia oleracea L. cv. Monatol). The membranes were frozen in a complex salt medium containing the predominant inorganic electrolytes of the chloroplast stroma. Under mild freezing conditions, only partial stabilization of PSII-dependent photosynthetic activities but efficient cryopreservation of PSI-mediated photophosphorylation took place in the presence of BSA. The lower the freezing temperature, the less protein was necessary for optimum protection of light-induced proton uptake and ATP synthesis. At higher initial BSA concentrations or more severe freezing, i.e. when the protein concentration in the residual unfrozen fraction exceeded a critical limit, rapid inactivation of photophosphorylation occurred in parallel with the ice crystal formation. This damage was due to an increase in the proton permeability of the membranes, while the capacity of linear whole-chain electron transport remained unaffected or even rose relative to unfrozen membranes. The effects of the solutes on thylakoid activities can be explained in part by colligative action, i.e. each solute acts on different membrane sites and reduces the concentration of the others in the residual unfrozen liquid. In addition, damage and protection of the membranes was influenced by the quantity of ice formed, the final volume of the unfrozen solution, the final membrane concentration in this fraction and the duration time of exposure to a given temperature. Extremely high concentrations of BSA and thylakoids in the frozen system caused inactivation of photophosphorylation regardless of the volume of the residual liquid and the freezing temperature. When chloroplast membranes were kept at 0 °C, BSA effectively stabilized cyclic photophosphorylation but only slightly reduced the progressive inactivation of linear photosynthetic electron flow.