Cumulative Risk Assessment (CRA) is defined in U.S. EPA's Risk Assessment Forum (RAF) CRA Framework (U.S. EPA, 2002a) as the combined risks from aggregate exposures to multiple agents or stressors. CRA has become an important research area, reflecting the interest of U.S. EPA's regional risk assessors, program offices, Office of Environmental Justice, and Office of Children's Risk. In implementing the requirements of the Food Quality Protection Act of 1996, U.S. EPA's Office of Pesticide Programs has developed guidance for conducting CRA's of chemicals that appear to act by a common mechanism of toxicity (U.S. EPA, 2002b). Because the organophosphorus pesticides (OPs) are considered to exert some of their toxic effects via a common toxicologic mechanism (i.e., cholinesterase inhibition), these compounds have been the subject of a CRA (U.S. EPA, 2001b). Additional CRA's may be performed on additional pesticide classes (e.g., triazinines, carbamates) and other co-occurring substances for which a common mode of action can be identified. The risk assessment method employed in the OP cumulative risk study is the Relative Potency Factor (RPF) approach (U.S. EPA, 2000). Dose addition is the critical methodological assumption, requiring the mixture components to act by the same toxic mode of action and to have similarly-shaped dose-response curves.