Summary 1. Under the conditions described, good viability of frozen mammalian cell cultures was often obtained following slow thawing in the presence of added di-methylsulfoxide. 2. Variation of pH of the freeze medium had no effect on viabilities of frozen HeLa, SJ. cells following slow or rapid thawing. 3. Dimethylsulfoxide is superior to glycerol in preventing damage of the cells tested during freezing. This is particularly evident when slow thawing is employed.