In response to specific treatment of vitamin B12 deficient, anaemic patients there is an influx of folate into the young, circulating red cells. To separate new and old cells, capillary tubes filled with whole blood were centrifuged and the packed red cell column divided into top (T), middle (M) and bottom (B) layer. The newest cells are found in the T layer. The increase in red cell folate (RCF) concentration starts before, during or after the reticulocyte response, and is therefore not directly related to folate metabolism in the red precursor cells in the marrow. The low RCF concentration at the peak of the reticulocyte response in some of the cases demonstrates that the folate material, which may have been accumulated in the red precursor cells in the marrow, may be lost by the time the red cells enter the peripheral blood. The influx of folate into the young, circulating red cells is rapidly followed by an efflux of folate, suggesting that much of the folate material is still in the monoglutamate form. A new influx of folate is noted after a time lapse of from 5 to 10 d. Iron deficiency seems to prevent the uptake of folate by the circulating red cells.