En(a-) and EnaEn red cells from different sources were studied using biochemical and serological methods. The results suggest that Finnish En(a-) erythrocytes contain only "N" but no M antigenic properties. Data on the members of the English En(a-) family suggest that English En(a-) red cells exhibit a combination of three rare alterations. The English En(a-) individuals are apparently heterozygous for the defects En and Ms, to all appearances, converted to a M ("M") antigen. These extraordinary data are brought to light by investigations on the children of the propositus (G.P. and J.P.), whose red cells are M-N+S-s+"N"+"M"- and M-N+S+s+"N"+"M"+ respectively. Sodium dodecylsulphate polyacrylamide gel electrophoretic results indicate that the MN glycoprotein content is decreased by about 50% in all English S+s+EnaEn red cells, the Ss glycoprotein being normal. G.P. erythrocytes, however, have only about half of the normal MN and Ss glycoprotein content.