Uptake of 5-methyltetrahydrofolic acid, and serum and red cell folate levels were measured in peripheral blood samples from seven mammalian species. Velocity of uptake of 5-methyltetrahydrofolic acid, at a concentration of 1 microM, increased from 0.001 pmole/10(9) red cells/minute (guinea pig) to 0.7 pmole/10(9) red cells/minute (pig) in the sequence: guinea pig less than sheep less than dog less than man less than rabbit less than rat less than pig. Michaelis constants (Km) were similar for human, dog and sheep erythrocytes (0.5--0.7 microM) but higher in red cells from rabbit, rat and pig (1.7--3.5 microM). Serum and red cell folate levels were comparable in human and dog samples; both levels were higher in rabbit and rat blood. In contrast, red cell folate levels were much lower in pig, sheep and guinea pig peripheral blood. These studies provide evidence that folate compounds are transported by an erythrocyte membrane carrier which is separate from the known permeation system for anions, since the species sequence for the latter is entirely different. They also suggest that some mammalian species, such as the dog, may be more relevant to man for studies of blood folate metabolism than several other species.