Swiss mice at different stages of pregnancy were treated intragastrically with the pesticide Carbendazim (MBC., BCM, methyl-2-benzimidazole carbamate) together with sodium nitrite. Lymphomas developed in 33.3% of young mice whose mothers were treated in the first week of pregnancy, in 53.3 % of those whose mothers were treated during the second week, and in 38.8% of those born of mothers treated during the third week. Treatment during the whole period of pregnancy yielded on an average 70.0% malignancy in offspring. However, administration of Carbendazim by itself did not produce lymphomas in the first generation. In lymphomas induced by in vivo-formed N-nitroso compounds, A- and C-type oncornavirus particles were observed with the electron microscope.