The effect of Rhizopus delemar on the carcinogenicity in rats of Aflatoxin B1 was studied. The Aflatoxin B1 was administered in free drinks to each male wistar rat at 126 micrograms per week such that a total dose of 3.40 mg was given over a period of 27 weeks. The culture abstract of Rhizopus delemar was added simultaneously to a group of these rats by mixing the Aflatoxin B1 solution. Animals were killed separately during 18th, 30th, 38th and 52nd week. Liver cell altered foci and neoplasms were qualified by using light microscopic and electromicroscopic morphology, by the morphometry and by the enzymic reactions. In the group of Aflatoxin B1 the incidence of hepatocellular carcinoma was 71%. In the group receiving Rhizopus delemar together with Aflatoxin B1, the hyperplastic foci and pathological enzymic foci were decreased at all times pointed and atd at termination, and in none of the rats had liver neoplasms appeared. The result of this experiment showed that the Rhizopus delemar has intensive capacity in inhibiting the toxic damage and carcinogenicity of the liver by the Aflatoxin B1, because it is not only able to postpone the appearance of altered foci and to control their development but also to accelerate their withdraw in advance. The Rhizopus delemar can be used as a feasible and efficacious means to control the intoxication of Aflatoxin B1.