Abstract Extracts of Aspergillus nidulans wild type ( bi-1) and the nitrate reductase mutant niaD-17 were active in the in vitro restoration of NADPH-dependent nitrate reductase when mixed with extracts of Neurospora crassa, nit-1. Among the A. nidulans cnx nitrate reductase mutants tested, only the molybdenum repair mutant, cnxE-14 grown in the presence of 10 −3 M Na 2MoO 4 was active in the restoration assay. Aspergillus extracts contained an inhibitor(s) which was measured by the decrease in NADPH-dependent nitrate reductase formed when extracts of Rhodospirillum rubrum and N. crassa, nit-1 were incubated at room temperature. The inhibition by extracts of A. nidulans, bi-1, cnxG-4 and cnxH-3 was a linear function of time and a logarithmic function of the protein concentration in the extract. The molybdenum content of N. crassa wild type and nit-1 mycelia were found to be similar, containing approx. 10 μg molybdenum/mg dry mycelium. The NADPH-dependent cytochrome c reductase associated with nitrate reductase was purified from both strains. The enzyme purified from wild-type N. crassa contained more than 1 mol of molybdenum per mol of enzyme, whereas the enzyme purified from nit-1 contained negligible amounts of molybdenum.