The first Caucasian (MD) shown to exhibit the low-frequency MNSs system antigen, Dantu was detected due to an increased tendency of erythrocytes to be aggregated by substances that promote red cell agglutination. The donor was found to exhibit a novel variety of the Dantu gene complex (DantuMD), as judged from biochemical, immunochemical, and serological studies. The glycophorin (GP) A level of MD's erythrocyte membranes were slightly decreased (about 17%) but GP B was not significantly different from normal. GP A and GP B of MD's cells were shown to carry M and N or S and s antigens, respectively, indicating that MD exhibits two genes encoding GP A and two genes encoding GP B. MD's cells contain a Dantu-, N- and s-specific GP B-GP A hybrid GP (molar ratio to GP A approx. 0.6:1.0). Partial amino-acid sequence analysis indicates that the structure of this molecule is rather similar to, or completely identical with, that of the hybrid GP in DantuNE erythrocytes. The residues 1-39 or 40-99 of the latter molecule correspond to the residues 1-39 of s-specific GP B and the residues 72-131 of GP A, respectively. Statistical evidence suggests that MD exhibits a single gene encoding the hybrid GP. Thus, MD appears to be heterozygous for a typical anti-Lepore type gene complex that seems to comprise genes for GP A, GP B, and the GP B-GP A hybrid. The diminished GP A level and a decreased galactose-oxidase labelling of the major membrane protein (anion channel protein, band 3) in MD's cells is in accordance with previous data suggesting that band 3 might form a complex with GP A and the Dantu-specific hybrid GP. This complex formation may be necessary for optimum incorporation of the latter molecules into the membrane.