Two different enzymes that metabolize lactate in the presence of oxygen, either to acetate plus CO2 (lactate 2-mono-oxygenase; Lmox) or to pyruvate plus H2O2 (lactate oxidase; Lox) were encapsulated in human and murine red blood cells (RBCs). Lmox shows a low affinity for lactate (Km 22 mM) and thus works at a low rate at the lactate concentrations found in hyperlactataemia (5-20 mM). Encapsulation of Lox provides a constant catabolic rate under the same range of blood lactate concentrations, but generates H2O2, which is toxic to the enzyme-loaded RBCs. Co-encapsulation of both enzymes at a ratio of 20 units of Lmox/unit of Lox results in significant rates of lactate metabolism over a wide range (1-30 mM) of lactate concentrations with modest methaemoglobin formation (5-8.5%) and normal cellular ATP concentrations (1.1-1.23 mM). In vitro experiments with [1-14C]glucose and [U-14C]glucose have shown that Lmox/Lox-loaded RBCs counteract the production of H2O2 by increasing the amount of glucose metabolized in the pentose phosphate pathway. In vivo attempts to prove the efficacy of these engineered RBCs in removal of blood lactate in mice have failed because of the high aerobic capacity and high lactate metabolism of these animals. However, the results obtained in vitro suggest that the encapsulation of lactate-catabolizing enzymes may be useful in the treatment of hyperlactataemia.