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Isolated tomato (Lycopersicon esculentum var. Kc 146) fruit mitochondria could be stored successfully in the frozen state without a cryoprotective agent if the mitochondria were frozen quickly by immersion in liquid nitrogen and later thawed quickly at 30 C. Criteria of freezing damage were rate of respiration, adenosine diphosphate to oxygen ratio, and respiratory control ratio. Marked reduction in respiration and loss of respiratory control occurred when mitochondria were transferred from liquid nitrogen to -5, -10, or -18 C for 15 minutes prior to thawing at 30 C. Dimethylsulfoxide (5%) prevented freezing damage when mitochondria were incubated at -5 C but did not prevent freezing damage at -10 or -18 C. Isolated tomato mitochondria show promise as a model system for studying the nature of freezing damage and the mode of action of cryo-protective agents.