Polycyclic aromatic hydrocarbons (PAHs) are one of the most ubiquitous classes of environmental carcinogens; however, limited information is available to describe their potential genotoxic interactions. This manuscript reports on the interactions of PAHs in complex mixtures as determined in microbial mutagenicity assays. Samples analyzed included separate 2-, 3-, and 4-ring PAH individual model fractions (IMFs) constructed to simulate the composition of a model coal tar. These were tested individually and in various combinations, including a reconstituted model fraction (RMF) composed of all three IMFs. A solvent extract of coal tar and a benzo(a)pyrene-amended extract of coal tar were also tested. The maximum mutagenic response of 1,089 revenants was induced by the RMF at a dose of 90 μg/plate with metabolic activation. At the four lowest dose levels, the response observed in the RMF sample was increased when compared to the 4-ring-IMF sample alone. However, the response observed with the RMF sample at the highest dose tested (900 μg/plate) was less than was observed in the 4-ring-IMF sample tested independently. When IMF samples were combined or mixed with individual chemicals, some inhibition was observed. These data indicate that mixtures of PAHs can exhibit a variety of mutagenic interactions controlled by both the metabolism of the PAHs and by their concentration in the mixture. This research was supported by NIEHS Grant No. P42-ES04917.