Data from 838 Finnish Landrace or Finnish Landrace crossbred sheep showed a highly significant correlation between phenotypes of the C blood group system and erythrocyte amino acid transport variants. Erythrocytes with normal amino acid transport properties (GSH high, Ly- type) were Cb-positive or Cb-negative. Erythrocytes with the amino acid transport lesion (GSH low, Ly +) were never Cb-negative. Sheep erythrocytes homozygous for Cb showed stronger lysis reactions with anti-Cb than heterozygous cells. Ly + sheep were nearly always homozygous for Cb, whereas most Ly- sheep were heterozygous for Cb-negative. Inheritance studies provided strong evidence that this association is due to close genetic linkage.