Montmorillonites are common anti-caking agents included in animal feeds that have been shown to adsorb aflatoxins in the gastrointestinal tract and diminish their bioavailability. Based on this evidence, it is hypothesized that clay-based enterosorption of aflatoxins may be a useful strategy for the prevention of aflatoxicosis in human populations. Long-term dietary exposure to NovaSil, a calcium montmorillonite clay, has been shown to be safe in animal models and human trials. Natural clay deposits may contain heavy metals and dioxins. Montmorillonites intended for human consumption, should not contain contaminant levels exceeding the tolerable values reported by JEFCA (Joint FAO/WHO Expert Committee on Food Additives). In a 3-month study in rodents, the mineral characteristics, aflatoxin sorption capacity and safety of refined NovaSil clay (UPSN) and Na-bentonite were investigated. Four-week-old male and female Sprague Dawley rats were fed rations free of clay (control) and containing UPSN and Na-bentonite at low dose (0.25%) and high dose (2.0%) levels for 13 weeks. AFB₁ sorption characteristics were the same for both clays. No treatment dependent differences in relative organ weights or gross appearance were observed due to the inclusion of clays. Analysis of hematological parameters, clinical chemistry, and selected vitamin and mineral levels revealed isolated significant differences between some treatments and the control group. Major findings resulting from the addition of clays were an increase in serum Na/K ratio, Na and vitamin E concentrations as well as a decrease in serum K of either males or females. However, none of these differences were dose-dependent and all parameters fell between ranges reported as normal for rats less than 6 months old. These results suggest that dietary inclusion of UPSN and Na-bentonite at levels as high as 2.0% (w/w) does not result in overt toxicity. Supported by USAID-PCRSP TAM149 and NIH 1R01MD005819-01.