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The low-temperature X-ray diffraction studies of orthorhombic insulin citrate crystals reported here were initially undertaken because of the rapid deterioration at room temperature of certain insulin crystals containing heavy-metal cations. The effects of cooling on mosaic character and radiation sensitivity were studied. Two temperature ranges were employed: (a) below -150'C and (b) 00C to -13'C.In normal laboratory practice orthorhombic insulin citrate crystals are grown and stored at 1 d 10C. Immersion studies in heavy-atom reagents are also made at this temperature. All X-ray diffraction work is, however, carried out at room temperature (.210C).We have found that orthorhombic insulin citrate crystals may be cooled rapidly to below -150'C without impairing the X-ray diffraction pattern. Rapid cooling does, however, enhance the mosaic character of these crystals. X-ray diffraction studies at 00C and -13'C have provided evidence for marked reduction in radiation sensitivity in certain heavy-atom-containing crystals.General Background.-Although metal-free insulin crystals are somewhat more sensitive to radiation damage (nickel-filtered CuKa) at room temperature than at 00C, the radiation sensitivity at room temperature is not marked enough to prevent the collection of intensity data by counter techniques. ' Furthermore, these crystals do not deteriorate on standing at room temperature for several weeks without irradiation.