Hereditary elliptocytosis is a heterogeneous disorder resulting from defects in the erythrocyte membrane skeleton. Although some cases of elliptocytosis result from defects in spectrin, the specific structural abnormality has yet to be identified in the majority of cases. Protein 4.1 plays an essential role in erythrocyte membrane physiology, and deficiencies have been implicated in only a few rare cases of elliptocytosis. By using 4.1 immunoblots and a 4.1 radioimmunoassay we identified distinct variants of protein 4.1 in 15 elliptocytic members of three US white families with the Rh-linked form of elliptocytosis. Elliptocytic members of family G were heterozygotes for a low-molecular weight (mol wt) 4.1 variant (65,000 to 68,000 daltons; normal, 80,000) inherited in linkage with the Rz phenotype. Elliptocytic members of family C expressed a simple partial deficiency of protein 4.1 (63% of the normal level) that was inherited in linkage with the r phenotype. Elliptocytic members of family N were heterozygotes for a high-mol wt 4.1 variant (100,000 daltons) also inherited in linkage with the r phenotype. These studies indicate that mutant forms of protein 4.1 are not uncommon in elliptocytosis among whites and that different kindreds probably express different mutations. The observed linkage of elliptocytosis and Rh blood type most likely results from the close proximities of the 4.1 gene (site of the mutation) and the Rh gene, which is located nearby on the short arm of chromosome 1.