Kx is a polytopic membrane protein of human erythrocytes carrying the Kx blood group antigen, which is deficient in rare patients with McLeod syndrome. Kx is disulphide bond linked to the Kell glycoprotein, which is a bitopic type II membrane protein carrying the Kell blood group antigen. Mice immunized with a synthetic peptide predicted to be located on the second external loop of Kx produced a monoclonal antibody called 3E12 which does not recognize red cells with common Kell phenotype by agglutination and flow cytometry. 3E12 recognizes the Kx protein and the spectrin beta-chain on western blots, the affinity for these two proteins being lowered with increasing ionic strength. Linear epitopes recognized by 3E12 are E116EIEKE121 and L484AQELEKE491 on the Kx protein and spectrin beta-chain, respectively. To quantify the relative amount of Kx in Empigen BB extracts of red cell membranes, an ELISA for Kx was set up which showed conclusively that (i) there is less Kx in membranes of K0 individuals (lacking the Kell glycoprotein) than in membranes of common individuals, and (ii) that all common individuals, typed as K+k-, K-k+ and K+k+, have the same amount of Kx on their red cell membranes. When an erythrocyte membrane detergent extract from one K0 individual was chromatographed on an immobilized 3E12 column, a minute amount of authentic Kell glycoprotein was recovered in acid eluted fractions, indicating that at least the K0 individual under study may still produce some Kell protein.