Nitrosoureas are among the most effective antineoplastic drugs in experimental models. New water-soluble analogues have been tested extensively in preclinical studies, and the results show that new congeners, such as 1-(2-chloroethyl)-1-nitroso-3-(2-hydroxyethyl)urea (HECNU), are more effective antineoplastic agents in most model systems than are clinically used compounds, such as 1,3-bis(2-chloroethyl)-1-nitrosourea or chlorozotocin. In chronic toxicity studies in rats, however, HECNU was found to be considerably less toxic than its clinically used congeners and was also less carcinogenic. Carbamoylating and alkylating potential and glutathione reductase inhibition do not parallel its antitumour effectiveness, chronic toxicity or carcinogenic potential. Compounds with higher anti-tumour activity in preclinical trials also show a higher potential for DNA-DNA interstrand cross-link formation in vivo. It is concluded that selection of analogues for clinical application should take into account not only antineoplastic effectiveness and acute toxicity, but also chronic toxicity and carcinogenicity.