Cryopreserved chimpanzee erythrocytes infected with Plasmodium falciparum were examined by electron microscopy directly and after a period of culture. Light microscopic observations on the viability of cryopreserved parasites in culture were also made. Parasitaemia data from a chimpanzee infected with parasites cryostored for more than 2(1/2) years are presented. Varying degrees of damage were apparent as the parasites were examined in vitro, for viability, and by electron microscopy. Schizonts appear not to survive the process, whereas the ability of "ring" trophozoite forms to develop into large trophozoites in vitro varies from 20% to 100% in different preparations. Cryopreserved material examined by electron microscope showed degenerative changes in most of the parasites, but after 72 h of cultivation most of the parasites appeared normal. These findings suggest that some organisms may sustain reversible damage during cryostorage.