Minced mackerel muscle was less stable at -20 °C than minced bluefish muscle as judged by odor evaluation but not by production of TBA-reactive substances. Minced mackerel muscle lost total glutathione faster than minced bluefish muscle at both -20 and 2 °C. Loss of glutathione was also more rapid in mackerel fillets than in bluefish fillets, but the differences were less. Sensory scores declined in both species after about two-thirds of the glutathione was lost. Neither soluble selenium nor glutathione peroxidase, oxidized glutathione reductase, or glutathione transferase activities appeared likely to account for the differences observed in glutathione losses. In a model system of washed, minced mackerel light muscle, glutathione/glutathione peroxidase was an effective antioxidant against lipid oxidation induced by an exogenous free radical-generating system. It seems possible that glutathione protected the fish muscle against quality loss in the early stages of storage.