The red blood cells of lambs, genotypically low potassium type, undergo a transition from high potassium to low potassium cell type from parturition onwards. This involves gradual changes in cell ion content, sodium pump activity, and ouabain binding. In the present study we investigated the properties of fetal red blood cells from 30 days prepartum using the chronically cannulated pregnant ewe preparation. We demonstrate that intracellular sodium increases and potassium decreases from -30 days onwards. Sodium pump activity monitored either by tracer potassium influx or ouabain binding is markedly higher in the early fetal samples examined and declines fourfold during the final month in utero. Unlike the maternal low potassium cells the early fetal red cells are refractory in terms of sodium pump stimulation by anti-L, the antibody in fact consistently inhibiting the pump. Finally, we have investigated the volume sensitivity and development of the ouabain-insensitive potassium fluxes in these cells and found that both fetal and maternal cells show a marked chloride-dependent, volume-sensitive passive potassium flux. We conclude that the decrease in active sodium transport between fetal red cells and adult low potassium cells is achieved partly by a reduction in the density of sodium pumps per cell, and then later by the introduction into the circulation of cells with Lp-antigen-modified sodium pumps.