A colorimetric assay, based on the hydroxylation of 4-nitrobiphenyl (NBP), allows for the first time the quick measurement of the biphenyl hydroxylase activity of whole liquid cultures of Aspergillus parasiticus. The assay can be used to measure the activity in whole cultures containing biphenyl hydroxylase substrates other than NBP. The assay was used to investigate the magnitude and time dependence of the activity induced by different culture treatments. The biphenyl hydroxylase activities measured by the assay correlate with the rates of biphenyl substrate hydroxylation as determined by chromatographic (HPLC) analysis, so the assay is a convenient alternative to the HPLC method for determining effects of culture conditions on hydroxylation rates. The assay was used to demonstrate that biphenyl substrates (4-bromobiphenyl and m-terphenyl) and a product of biphenyl hydroxylation (4,4’ -dihydroxybiphenyl) induce (i.e., stimulate synthesis of) the hydroxylase system. The rates of hydroxylation of NBP observed in the assays (up to 0.6 g product/1 per hour) are considerably higher than the rates of biphenyl substrate hydroxylation previously reported in cultures of A. parasiticus.