Pyruvate dehydrogenase is a multienzyme complex consisting of three enzymes, a decarboxylase (E1l), an acetyltransferase (E2) and a dehydrogenase (E3). A fourth enzyme of unknown function component X or protein X has recently been identified. Several similarities between E2 and protein X have been demonstrated. Further investigation into this unique polypeptide has shown monospecificity of antiserum raised against gel purified protein X and E2. In various cell lines, antiserum to protein X failed to recognise the E2 component, similarly with E2 antiserum no cross reactivity with protein X polypeptide was observed.