To date, few researchers have assessed the toxicities of Polycyclic Aromatic Hydrocarbons (PAH's)as mixtures, although that is how they commonly occur in the environment. The toxicities of 2, 3, and 4 ring PAH's were determined for individual compounds. Synergistic antagonists or additive effects of mixtures of two, three, and four PAH compounds were assessed by comparing the toxicities of those mixtures to that of individual compounds. Microtox® mixture results were also compared to those predicted by the Σ PAH model. The toxicities of chrysene, pyrene, phenanthrene, fluorene, fluoranthrene, acenaphthalene, phenanthrene, and naphthalene were evaluated using the Microtox® acute and chronic toxicity assay which uses a bioluminescent bacteria, Vibrio fisheri, to measure toxicity. Results indicate that, as individual compounds, the acute toxicities of PAHs are inversely related to water solubility. The research also showed that mixtures can be synergistic, antagonistic, or additive. Chronic tests showed that PAHs with lower water solubility require greater concentrations than other PAHs to show a toxic response. Lastly, from the research, one can see that the additive concept for PAH mixtures is generally upheld.