Fetal mice exhibit two types of red blood cell lineages: primitive and definitive. Primitive mouse red blood cells lacking erythroid Krüppel-like factor (EKLF) exhibit abnormal morphology and disappear before achieving the biconcave shape of normal primitive blood cells. To explore whether a mechanical instability in the cell surface contributes to this phenomenon, the cell membrane stability of the EKLF -/- red blood cells was measured using a bioforce probe technique and compared to the properties of primitive red blood cells from normal embryos. In this technique, a long cylinder of membrane bilayer (tether) was formed between the primitive cell surface and a bead. The process was observed in a light microscope, and the force required to form the tether was measured. This force depends on the mechanical tension in the membrane and the natural tendency of the membrane bilayer to remain attached to the cell surface. Our preliminary data show that normal primitive blood cells and primitive EKLF -/- blood cells both exhibit heterogeneous properties similar to those of maturing red blood cell populations of the definitive lineage.