Fourteen serologically distinct mycoplasmas from various avian and mammalian sources were stored frozen in liquid medium. At -70°C, the number of viable organisms of only three of these mycoplasmas decreased by 1 log10 after 3 1/2 yr. Four of the same mycoplasmas of human origin were stored at -50°C and the number of viable organisms remained undiminished after 2 yr. On the other hand, at -30°C the viability titres of 8 of 11 mycoplasmas decreased by at least 1 log10, but by no more than 2 log10 after a similar period of time. Lyophilisation caused an immediate decrease in the number of viable organisms of most but not all mycoplasmas so treated. The addition of several “stabilising” fluids, used in the lyophilisation of viruses and bacteria, was not helpful. These had no immediate protective effect and some of them rapidly killed the organisms during subsequent storage. Cultures of ten mycoplasmas in liquid medium containing 2 per cent. bovine plasma albumin were lyophilised and samples of each lyophilised culture were stored in parallel at 37°C and at 4°C. They all contained viable organisms after storage at 37°C for 15–18 mth; of 7 cultures tested after 27–34 mth, 4 of them still contained viable organisms. All the lyophilised cultures stored at 4°C contained viable organisms, seven of them in undiminished numbers, after 27–34 mth.