Quite especially preferred are nitrosobenzene derivatives of the general formula: ##STR2## wherein R is a hydroxyl or amino group, the amino group being optionally substituted one or more times by lower alkyl radicals and the lower alkyl radicals can, in turn, be substituted by a hydroxyl group, an amino group mono- or polysubstituted by lower alkyl radicals, PO3 H2, SO3 H or COOH.