Over the last 5 years, the ATP cell viability assay (ATP-CVA) has been used to study the in vitro response of cell lines and fresh gynecologic human tumors to a variety of antineoplastic agents including chemotherapeutic agents, hormones and biological response modifiers. This assay measures light production as intracellular ATP interacts with the luciferin-luciferase complex. Quantitation of the light produced has been shown to directly correspond with the number of viable cells. A past criticism is that in the ATP-CVA, when applied to fresh tumor tissue, normal cells (fibroblasts, macrophages and lymphocytes) also produce ATP, and if present in sufficient numbers, could lead to errors in chemosensitivity testing results. This study was designed to evaluate the growth characteristics of various benign cells found in fresh tumors. The cells were studied under multiple plating conditions to show the relative increase or decrease of fractional ATP measured at different time points. We found that agar/McCoy underlayer and agarose-coated wells do not permit the growth of nonmalignant cells. In the culture conditions of the ATP-CVA, nonmalignant cells do not contribute relevant ATP levels when treated samples are compared to controls on day 6. Therefore, results of the ATP-CVA in fresh tumors should not be affected.