Plasmodium falciparum-parasitized erythrocytes obtained from infected owl monkeys (Aotus trivirgatus) and stored in the frozen state retain both the capacity to incorporate 14C isoleucine into protein and to infect animals. The cryopreservation method involves the use of glycerol and the reconstitution of isotonicity after thawing. Animals have been infected from material held for up to 273 days and protein synthesis has been demonstrated in vitro for up to 180 days after freezing. The specificity of protein synthesis as an activity of the parasites was shown by the inactivity of control uninfected erythrocytes stored by the same method. Additional evidence for the specificity of the reaction was obtained from inhibition studies with chloroquine; a 7 X 10(-5) M concentration of the drug resulted in 50% inhibition of the initial rate of protein synthesis.