Nondialyzable Maillard reaction products (MRPs) were recovered from three different coffee brew extracts (i.e., brewed, boiled, instant) to evaluate the efficacy of MRPs in modulating in vitro metal-induced cytotoxicity in C3H/10T1/2 mouse embryo fibroblast cells, cultured in the presence of Fe2+, Fe3+, or Cu2+ ions. Preliminary experiments were performed in an vitro linoleic acid emulsion model system to characterize the anti- or pro-oxidant activity of coffee MRPs. Cytotoxicity experimental protocols involved both the direct application of metal ions and coffee MRPs to fibroblast cells, and the premixing of metal ions with coffee MRPs at room temperature prior to incubating with fibroblast cells. Fe2+ and Cu2+ significantly lowered the colonization efficiency (CE) of cells at all three concentrations (i.e., 0.1, 10, 50 mu M) used. Similar Fe3+ activity was observed only at 50 mu M concentration. None of the coffee MRPs alone or together with 0.1 and 10 mu M of Fe2+ or Fe3+ produced cytotoxic effects during direct application. The premixing step, however, significantly enhanced the CE of cells compared to the control, denoting cytoprotection, only in the presence of Fe2+. In addition, the application of MRPs with 0.1 or 10 mu M of Cu2+ significantly lowered the CE of cells than the control, but enhanced the CE of cells than the Cu2+ added control. These results corresponded directly with the results of model linoleic acid emulsion test, thereby demonstrating that lipid hydroperoxide generation is the source for fibroblast cell toxicity when MRPs are added to cells together with metal ions. These results further indicate that coffee MRPs can suppress in vitro metal-induced cytotoxicity to a certain extent when Fe2+, Cu2+, or Fe3+ ions are present below a concentration of 50 mu M , possibly by chelating the metal ions. Ionic reducing capacity of coffee MRPs, albeit small, may explain the potential for increased cytotoxicity at higher coffee MRP concentrations.