Abstract Methyl-(β-tosyloxyethyl) nitrosamine and 3-methyl-4, 5-dihydro-1, 2, 3-oxadiazolium tosylate are potent direct acting mutagens in the Ames assay, as is N-nitrosoprolinyl tosylate. These compounds are derived from β-hydroxylated nitrosamines. The closely related methyl-