Material which contains water, or is accompanied by an aqueous phase, notably biological cells, cell components or cell aggregates, or differentiated biological tissue is preserved by dispersion in an oil medium and under-cooling the dispersion, preferably to a temperature in the range -20 DEG C. to -40 DEG C. The oil medium is characterized by the absence of surfactant which can catalyze ice formation and is an immobile gel at the storage temperature. The preferred oil 10 medium is paraffin oil, or oil plus paraffin wax.
