The cnx- group of mutants of Aspergillus nidulans lacks xanthine dehydrogenase (xanthine: NAD+ oxidoreductase, EC 1.2.1.37) and nitrate reductase (EC 1.6.6.3) activities and are thought to be defective in the synthesis of a molybdenum-containing cofactor, 'cnx', common to xanthine dehydrogenase and nitrate reductase [Pateman, J.A., Rever, B.M., Cove, D.J. and Roberts, D.B. (1964) Nature (Lond.) 201, 58-60]. The cnx cofactor has a role in maintaining the aggregated multimeric structure of nitrate reductase [MacDonald, D.W., Cove, D.J. and Coddington, A. (1974) Mol. Gen. Genet. 128, 187-199]. We report here that, in cnx- mutants grown under conditions inducing xanthine dehydrogenase I, a species cross-reacting with antisera to the native enzyme and of half its molecular weight is present, together with cross-reacting molecules of similar molecular weight to the native enzyme. This suggests that the cnx cofactor has a role in maintaining the aggregated structure of xanthine dehydrogenase I. Both cross-reacting species are capable of passing reducing equivalents from NADH to a tetrazolium salt, showing that the cnx cofactor is not necessary for enzymic activity towards NADH.