Sodium ascorbate can be used as a preservative of patient samples for folate assay when freezing of serum is impractical. To evaluate the effect of sodium ascorbate on folate levels in human serum, it was added to pooled human sera in 1 g/l increments from 0 to 10 g/l serum. Free folate levels remained constant when the sodium ascorbate concentration was 6 g or less per liter of serum. At more than 6 g/l serum, free folate levels decreased. Bound folate levels increased when sodium ascorbate levels were 4 g/l or less, but remained stable at more than 4 g/l. A sodium ascorbate concentration of 5.0 +/- 1 g/l serum provided optimal preservation of folate in patient samples, indicated by obtaining constant values for four days when serum was kept at room temperature.