Recessively inherited ovalocytosis in coastal Melanesians is associated with widespread, but selective, depression of blood group antigens in homozygotes, who comprise about 15% of these populations. It is suggested that a membrane anomaly exists, and that the series of depressed determinants all depend, for their full expression, upon the same membrane component(s), the proper synthesis of which is being genetically affected. Affected antigens would thus be associated by position and/or structure. Antigens subject to depression include IT, IF, LW,D,C,e,S,s,U,Kpb,JKa,JKb,Xga,Wrb,Scl and Ena, which are simultaneously affected when present on oval cells. Reactivity of A1,ID,i,P1,M,N,Lub,k,Fya,Coa, Vel and Gea appears to be within the normal range on depressed cells, as does sialic acid content. Indirect evidence suggests that Z (associated with S and s) is among the depressed series, while NA and Leb are not. High H variants, common among Melanesians, seem absent among depressed bloods, and when such variants were excluded, H, A and B did not appear subject to depression. The distribution pattern of ovalocytosis suggests that it may confer some selective advantage in the tropical coastal environment. The findings also have implications concerning population genetic work in New Guinea.