Endogenous superoxide dismutase (SOD) has not been found to be present in P. vivax, a human malarial parasite and therefore it adopts and concentrates SOD from the host cell erythrocytes. It is demonstrated here that this adopted SOD from the host gets localized in lysosomes (10 k and 100 k fractions) of the malarial merozoites. P. vivax parasites were also found to contain very low levels of catalase, presumably as a result of contamination or adoption from the host red cell materials. It is therefore suggested that P. vivax merozoites are deficient in enzymes which are protective against the reactive oxygen species.