Oral leukoplakias, particularly non-homogenous types, are often invaded by yeasts, withCandida albicansbeing the dominant species. The more advanced precancerous leukoplakia lesions yield more rarely occurring biotypes of C.albicans, suggesting a causal role for these biotypes in the malignant transformation.N-nitroso-benzylmethylamine (NBMA) is a compound able to induce carcinoma of the esophagus and the oral cavity in the rat. The catalytic potential of yeasts, isolated from leukoplakia lesions and from normal mucosa, to produce NBMA from the precursorsN-benzyl-methylamine and nitrite was assessed at pH 6.8. The yeast strains differed in nitrosation potential, ranking from 0 to 1.2μg NBMA/106cells.C. albicansstrains of the more rarely occurring biotypes showed the highest nitrosation potential, whereasC. tropicalis, C. parapsilosis, andTorulopsis glabratawere ranked lower. Strains with high nitrosation potential were generally isolated from lesions with more advanced precancerous changes. Thus, further evidence is provided supporting the hypothesis that yeasts play a causal role in oral cancer by means of endogenous nitrosamine production.