The interaction of sodium selenite (Na2SeO3) and aflatoxin B1 (AFB1) was studied in 6-wk-old male Mongolian gerbils. Each of four groups of gerbils were fed one of the following diets during a 12-wk experimental period: control (commercial Chow), 5.0 ppm Na2SeO3, 12.8 ppm AFB1, or 5.09 ppm Na2SeO3 + 12.8 ppm AFB1. Animals receiving Na2SeO3 in the diet, alone and with AFB1, had a significantly lower mean total weight gain during the experiment than did control animals. Animals receiving both compounds together displayed a very high level of physical activity compared to the three other groups. Blood analysis showed no change in total leukocytes, but the relative percentage of lymphocytes increased and the percentage of neutrophils decreased concurrently in the order: control less than AFB1 less than Na2SeO3 + AFB1 les than Na2SeO3. A significant reduction in organ weight relative to body weight was observed in the liver, kidney, and lung of the animals fed AFB1 alone but only in the liver of those fed both Na2SeO3 and AFB1. No similar alterations were observed in the Na2SeO3 group. Histopathological examination revealed considerably less hepatic damage in animals fed Na2SeO3 with AFB1 than in those receiving either compound alone. Renal and intestinal damage, however, was most severe in this double-treatment group. Hepatic protein analysis revealed two protein peaks in the Na2SeO3 + AFB1 group that were absent in all other groups. It was concluded that these proteins may be selenoproteins directly or indirectly involved in the lower incidence of histopathological damage in this group.