Red blood cell gamma-glutamylcysteine synthetase (GC-S) activities and glutathione (GSH) concentrations were investigated in pure-bred Finnish Landrace, Tasmanian Merino, and Finnish Landrace X Tasmanian Merino sheep. Previous studies were confirmed that a dominant gene (L) at the GSH locus leads to partial GC-S and GSH deficiency, while a recessive gene (h) at the Tr locus codes for defective red cell amino acid transport, concomitant GSH deficiency, and significantly elevated levels of GC-S. In addition, new results are presented which suggest that there is a dominant gene (A) at another locus distinct from GSH and Tr (provisionally designated GC-S) which has a marked epistatic enhancing effect on measured GC-S activities.