An inherited amino acid transport deficiency results in low concentrations of glutathione (GSH) in the erythrocytes of certain sheep. Earlier studies based on phenotyping according to GSH concentrations indicated that the gene TrH, which controls normal levels of GSH, behaves as if dominant or incompletely dominant to the allele Trh, which controls the GSH deficiency. The present papers shows that when sheep are classified according to amino acid transport activity, the TrH gene behaves as if codominant to Trh. Erythrocytes from sheep homozygous for the TrH gene exhibit rapid saturable L-alanine influx (apparent Km, 21.6 mM; Vmax, 22.4 mmol/liter cells/hr.). Cells from sheep homozygous for the Trh gene exhibit slow nonsaturable L-alanine uptake (0.55 mmol/liter cells/hr at 50mM extracellular L-alanine). Cells from heterozygous sheep show saturable L-alanine uptake with a diminished Vmax (apparent Km, 19.1 mM; Vmax, 12.7 mmol/liter cells/hr). These erythrocytes have a significantly lower GSH concentration than cells from TrH, TrH sheep but similar intracellular levels of dibasic amino acids.