The present invention relates to a mutant lactate oxidase having increased stability, a nucleic acid encoding the mutant lactate oxidase, an expression vector comprising the nucleic acid, a host cell comprising the nucleic acid or the expression vector, a method of determining lactate in a sample, the use of the mutant lactate oxidase for determining lactate, a device for determining lactate in a sample using the mutant lactate oxidase and a kit for determining lactate comprising the mutant lactate oxidase.