Human erythrocyte transmembrane sialoglycoprotein, glycophorin C, plays a functionally important role in maintaining erythrocyte shape and regulating membrane material properties, possibly through its interaction with protein 4.1. Moreover, it has previously been shown that membranes deficient in protein 4.1 exhibit decreased content of glycophorin C. To further define the relationship between protein 4.1 and glycophorin C, a series of studies were performed using both protein 4.1- and glycophorin C-deficient erythrocytes. Quantitation by flow cytometry showed that the glycophorin C content of cells totally deficient in protein 4.1 was 9% of normal and that of cells partially deficient in protein 4.1 was 44% of normal. Interestingly, while homozygous glycophorin C-deficient cells had no detectable levels of this sialoglycoprotein, cells from obligate heterozygotes had normal levels. Protein 4.1 content of membranes of these glycophorin C- deficient cells was also normal. These data suggest that glycophorin C may be synthesized in excess by erythroid cells and its membrane content regulated by protein 4.1. To investigate if this regulation is due to association between protein 4.1 and glycophorin C, we examined the retention of glycophorin C in membrane skeletons (Triton shells) prepared from normal membranes, protein 4.1-deficient membranes, and protein 4.1-deficient membranes reconstituted with exogenous protein 4.1. Glycophorin C is retained by Triton shells prepared from normal membranes, whereas Triton shells prepared from protein 4.1-deficient membranes are totally devoid of this sialoglycoprotein. However, reconstitution of protein 4.1-deficient membranes with purified protein 4.1 resulted in retention of glycophorin C with the Triton shells. This finding suggests that protein 4.1 is necessary for association of glycophorin C with the membrane skeleton. Furthermore, these data suggest that through its interaction with glycophorin C, protein 4.1 may play a role in regulating the membrane content of this sialoglycoprotein in mature human erythrocytes.