The objective of this research program was to develop an enzymatic screen for chemical carcinogens based on the selective in vitro stimulation of microsomal biphenyl-2-hydroxylase by known chemical carcinogens. An attempt was made to repeat published work using a spectrophotofluorometric assay for biphenyl metabolities. It was found that this assay system is not valid for use with complex mixtures, and that metabolites must be separated from interfering compounds prior to quantitation. A high pressure liquid chromatography method was developed which permitted rapid separation of metabolites. Nanogram quantities of metabolites were detectable using this chromatographic separation in conjunction with a spectrophotofluorometric detector. Using this method, it was not possible to demonstrate in vitro stimulation of biphenyl-2-hydroxylase by chemical carcinogens. Alternative assays were also examined. Terphenyl is metabolized to at least three different compounds by hamster microsomes. Further work is necessary to validate the utility of this substrate in an enzymatic screen for carcinogens. A marine protozoan, Parauronema acutum metabolizes biphenyl in vivo to 2- and 4-hydroxybiphenyl. This organism may provide a reliable, inexpensive source of biphenyl hydroxylase for an in vitro enzymatic assay system.