Seedlings of Triticum aestivum L. cv. Lennox were grown in different environments to obtain different hardiness. Pieces of laminae and leaf bases were slowly cooled to sub-zero temperatures and the damage caused was assessed by an ion-leakage method. Comparable pieces of tissue were slowly cooled to temperatures between 2° and-14°C and were then freeze-fixed and freeze-etched. Membranes generally retained their lamellar structures indicated by the abundance of typical membrane fracture faces in all treatments, and some membrane fracture faces had patches which lacked the usual scattering of intramembranous particles (IMP). These IMP-free areas were present in the plasma membrane of tissues given a damaging freezing treatment, but were absent from the plasma membrane of room-temperature controls, of supercooled tissues, and of tissues given a non-damaging freezing treatment. The frequency of IMP-free areas and the proportion of the plasma membrane affected increased with increasing damage. In the most damaged tissue (79% damage; leaf bases exposed to-8°C), 20% of the plasma membrane was IMP-free. The frequencies of IMP at a distance from the IMP-free areas were unaffected by freezing treatments. There was a patchy distribution of IMP in other membranes (nuclear envelope, tonoplast, thylakoids, chloroplast envelope), but only in the nuclear envelope did it appear possible that their occurrence coincided with damage. The IMP-free areas of several membranes were sometimes associated together in stacks. Such membranes lay both to the outside and inside of the plasma membrane, indicating that at least some of the adjacent membrane fragments arose as a result of membrane reorganization induced by the damaging treatment. Occasional views of folded IMP-free plasma membrane tended to confirm this conclusion. The following hypothesis is advanced to explain the damage induced by extracellular freezing. Areas of plasma membrane become free of IMP, probably as a result of the freezing-induced cellular dehydration. The lipids in these IMP-free patches may be in the fluid rather than the gel phase. The formation of these IMP-free patches, especially in the plasma membrane, initiates or involves proliferation and possibly fusion of membranes, and during or following this process, the cells become leaky.