Urban air particulate matter, collected from Washington, D.C. and certified by the National Institute of Standards and Technology (NIST) as Standard Reference Material 1649, was extracted and fractionated into acid, base and neutral fractions. Each fraction was tested for biological activity using a microbial mutagenesis assay system. The organic acid fraction showed unexpectedly high mutagenic activity, and was subjected to chemical characterization studies. Following derivatization, analysis by GC/MS showed the presence of fatty acids, aromatic acids (including phenolic compounds), and a significant number of compounds that could not be identified from mass spectral compendia. Spectroscopic and elemental analysis data supported the characterization of the fraction as predominantly aromatic. Mass spectra from both GC/MS and direct probe analysis showed the presence of a chlorinated substance, subsequently identified as the fungicide Dichlorophen. The compound was shown to comprise over 50 percent of the mass of the organic acid fraction. A reference standard of Dichlorophen was not mutagenic. The presence of the fungicide in the NIST certified urban aerosol is, in all probability due to artifactual processes. Attempts to concentrate the observed mutagenic activity by preparative chromatography and acid/base partition experiments were not successful. (Copyright (c) 1990 Gordon and Breach, Science Publishers, Inc.)