Urban air particles contain extractable organic matter which has both mutagenic and carcinogenic activity. Detection of the specific compounds responsible for mutagenic activity has been limited by the complexity of these extracts and the presence of highly polar components which have been difficult to identify. To facilitate identifications, these extracts must be separated into substantially less complex fractions and analyzed with newer analytical techniques. Bioassay-directed fractionation has been proposed as the most cost and time effective approach to identifying mutagenic compounds in complex mixtures by directing the analytical and identification research effort toward only significantly mutagenic fractions. This approach should be especially useful for the detection of heretofore unknown constituents of the matrix as the methods are not directed to or developed for specific compounds or compound classes.