There is a growing body of evidence that various pesticides and heavy metals are carcinogenic. If not directly, there is also evidence that shows that these compounds can participate in carcinogenesis in a passive or permissive role, facilitating other compounds from inducing tumor formation. Little evidence is available to aid in understanding the toxicity of metal-pesticide mixtures. In many instances, exposure to subclinical, or subtoxic, levels would be asymptomatic under a single-chemical exposure. But, we do not know how these compounds would act together. A synergistic or potentiating response could be highly possible. By chemically interacting with the environment, as well as each other, metal pesticide mixtures may yield unpredictable toxicity. Because we are not exposed to a single xenobiotic at a time, the importance of studying the toxicity of mixtures has never been more critical.