Vinyl chloride (VCM) is not carcinogenic by itself, it is bioactivated to the highly reactive alkylating oxirane chloroethylene oxide. Further metabolism, apparently, leads via an interaction of the primary alkylating metabolites with glutathion to S-(2-carboxy-methyl)-cysteine and thiodiacetic acid which are eliminated with the urine. Up to now, it has not been ascertained whether the oxirane alone is the essential carcinogenic factor or whether other metabolites are also involved in carcinogenicity. Likewise, it is still unknown whether the metabolites excreted in the urine might be used as biological criteria for exposure to VCM, because these metabolites probably can originate from a series of substances other than VCM. This problem could stimulate investigations on the possible carcinogenic activity of these substances.