A patient with a mild hemolytic anemia and osmotically fragile, spherocytic erythrocytes was studied. Analysis of the erythrocyte membrane proteins by SDS-PAGE revealed a deficiency of protein 4.2 (less than 0.10% of normal). The protein 4.2-deficient erythrocytes contained normal amounts of all other membrane proteins, although the amount of band 3 was slightly reduced and the amount of band 6 (G3PD) was slightly elevated. The spectrin content of these cells was normal, as measured by both SDS-PAGE and radioimmunoassay. Erythrocytes from the patient's biologic parents were hematologically normal and contained normal amounts of protein 4.2. Immunological analysis using affinity purified antibodies revealed that the patient's protein 4.2 was composed of equal amounts of a 74-kD and 72-kD protein doublet, whereas the normal protein was composed primarily of a 72-kD monomer. Proteolytic digestion studies using trypsin, alpha-chymotrypsin and papain demonstrated that the patient's protein 4.2 was similar but not identical to the normal protein. Binding studies showed that the protein 4.2-deficient membranes bound purified protein 4.2 to the same extent as normal membranes, suggesting that the membrane binding site(s) for the protein were normal. Depleting the protein 4.2-deficient membranes of spectrin and actin resulted in a loss of nearly two-thirds of the membrane ankyrin, whereas similar depletion of normal membranes resulted in no loss of ankyrin. Repletion of the protein 4.2-deficient membranes with purified protein 4.2 before spectrin-actin extraction partially prevented the loss of ankyrin. These results suggest that protein 4.2 may function to stabilize ankyrin on the erythrocyte membrane.