Myelotoxicity parameters were monitored in female B6C3F1 mice exposed to 0, 1, 5, and 10% of a chemical mixture stock in drinking water for 2.5 to 31.5 weeks. The mixture consisted of 25 groundwater contaminants frequently found near toxic waste dumps, as determined by U.S. Environmental Protection Agency (EPA) surveys. Water consumption, body and organ weights, and hematological and histopathological examinations were conducted. No animals developed overt signs of toxicity after 2.5 weeks of treatment. No significant effect on bone marrow cellularity was observed after 2.5, 15.5, or 31.5 weeks of exposure; however, mice exposed to 5% or higher concentrations of the chemical mixture stock solution for 15.5 weeks showed significant suppression of granulocyte-macrophage progenitor cells (CFU-GM) and erythroid precursors (CFU-E) with no changes in body weight, histopathological or hematological parameters. Decreases occurred in erythrocyte mean corpuscular volume of mice exposed to a 10% solution for 15.5 weeks and to 5 and 10% solutions following 31.5 weeks of treatment. In addition, dose-related decreases were found in body, liver, and thymus weights in the 5 and 10% solutions exposure groups after 31.5 weeks. These results suggest that bone marrow may be a sensitive indicator for long-term, low-level exposure of multiple chemicals in mice. Furthermore, long-term exposure to highly contaminated groundwater may present a subtle risk to the hematopoietic system.