The practice of cold storage for preserving labile material of biological origin is widespread. The general utility of this method and the successful preservation of living cells and tissues in the frozen state has overshadowed the fact that freezing can be a harmful process to living cells (Wood 1956). It used to be thought that the crushing or spearing action of ice crystal growth was the principal source of damage by freezing; indeed so reasonable is this theory that it is difficult to believe that some at least of the harmful effects of freezing are not due to this cause. The development of the theories of damage by ice crystal growth have been described in detail by Luyet & Gehenio (1940), and by Meryman (1956). By contrast with damage on a macroscopic scale which might occur during the growth of ice crystals there is evidence to show that freezing can damage the molecular constituents of living cells, and this is most unlikely to be a direct consequence of the intrusion of ice crystals. This aspect of the problem of freezing damage forms the basis of this paper.