The addition of Fluosol-DA followed by carbogen breathing increased the antitumor effect of cyclophosphamide as measured by both tumor growth delay and tumor cell survival assays. Under air breathing condition, cyclophosphamide (100 mg/kg) administered i.p. five times on alternate days produced a tumor growth delay in the FSaIIC fibrosarcoma of 8.0 +/- 0.8 days. Adding Fluosol-DA (0.3 ml) to treatment with cyclophosphamide followed by carbogen breathing increased tumor growth delay to 11.4 +/- 3.6 days, which was not statistically significantly different from that obtained with the drug plus carbogen breathing without Fluosol-DA. As the dose of Fluosol-DA was increased and administered with drug treatment followed by carbogen breathing for 6 h, increasing tumor growth delays of 15.0 +/- 1.5 days, 18.1 +/- 1.7 days and 29.4 +/- 2.2 days were observed with 0.1 ml, 0.2 ml and 0.3 ml Fluosol-DA, respectively. When 0.1 ml Fluosol-DA was administered in combination with cyclophosphamide and immediately followed by 1 h of hyperbaric oxygen (3 atm), a tumor growth delay of 13.7 +/- 1.2 days was observed. With 0.2 ml Fluosol-DA under these conditions, the tumor growth delay increased to 23.2 +/- 1.6 days, and with 0.3 ml Fluosol-DA the tumor growth delay was 35.6 +/- 3.2 days. Single doses of cyclophosphamide with and without Fluosol-DA (0.3 ml) and various conditions of oxygenation were used in an FSaIIC fibrosarcoma tumor cell survival assay. The addition of Fluosol-DA to this single-dose protocol produced a five- to tenfold increase in tumor cell kill compared to air-breathing drug-treated animals. There was no significant difference in the toxic effect of any of the treatment conditions on bone marrow.