We report the results and discuss the toxicity of clinical trials based on a single concept: the decrease in O6alkyl DNA alkyltransferase (O6AT) resistance mechanism when a chloroethylating agent is used sequentially after a methylating agent. This decrease in O6AT being dose dependent, several increasing doses of dacarbazine (DTIC) have been tested (400 mg/m2 to 1000 mg/m2 every 4 weeks, 3-4 h before fotemustine (100 mg/m2 intravenously every 4 weeks). These results (mean overall response rate 27%) compared with reference regimes, demonstrate that DTIC is able to increase the alkylating power of fotemustine: same range of response rate with only half of the two drug doses compared to an alternated combination, high activity rate especially in lung metastases (10/42 complete responses + 13/42 partial responses), different pattern for haematotoxicity, and occurrence of a new side-effect: acute lung toxicity as adult respiratory distress syndrome (ARDS). This lung toxicity was totally unexpected since several hundreds of patients had been so far treated with fotemustine as single agent or in other combinations with DTIC without any case of acute or delayed lung toxicity. Prophylactic administration of corticoids was not effective and monitoring of the respiratory function was of no predictive value. Due to the additional depleting effects of DTIC on at least two main defence mechanisms--the O6AT system and cytosolic and/or nuclear glutathione--we suppose that the sequence is able to increase the alkylating power of fotemustine to an excessive extent and/or that the detoxication capacity of the cell against DTIC and/or fotemustine metabolites is overwhelmed. Other depletors of the O6AT activity which do not generate metabolites that compete for the same detoxication pathway as the chloroethylnitrosourea (CENU) metabolites should be tested.