SR-2508, a less lipophilic ane neurotoxic analogue of the nitroimidazole, misonidazole, has exhibited significant chemosensitization properties in preclinical studies with alkylating agents. A phase I trial was carried out to assess toxicity and possible pharmacological interactions of the combination of short infusions of SR-2508 and cyclophosphamide (CP). Patients were randomly assigned to receive either CP alone followed in 3 wk by CP + SR-2508, or CP + SR-2508 followed by CP alone. All additional courses were CP + SR-2508. The maximum tolerated dose of the combination was determined by dose escalation of SR-2508 while the dose of CP remained fixed, initially 1.0 g/m2, and then a second maximum tolerated dose was determined with CP at 1.6 g/m2. One hundred seventeen evaluated courses were administered to 39 patients, the majority of whom had received prior treatment. Somewhat unexpectedly, reversible grade 4 granulocytopenia was the dose-limiting toxicity occurring in four of five evaluable first combination courses at level 6 (SR-2508, 11.3 g/m2; CP, 1.0 g/m2), the initial maximum tolerated dose. SR-2508 enhanced CP-induced myelosuppression as exhibited by the significant difference (p less than 0.001) between the 27 paired courses (CP versus CP + SR-2508) for WBC nadirs over levels 1 to 6. The neurotoxicity encountered was similar to that seen in past clinical trials, being reversible, mild, and usually peripheral in nature. There was one treatment-related death (neutropenic sepsis) on study. No other significant toxicity was seen. SR-2508 exhibited linear pharmacokinetics over the dose range studied. The SR-2508 area under the concentration-time curve increased linearly with dose (r = 0.858; p less than 0.001). No other parameters were dose related. Neither drug appeared to affect the pharmacokinetics of the other, and CP pharmacokinetic values were consistent with those from prior studies. Due to the interaction noted between the two agents and the preclinical data suggesting preferential enhancement of antitumor efficacy under this combination, phase II study appears warranted.