The present study was designed to analyse the cytotoxic effects of the combination of fotemustine plus 5-fluorouracil (5-FU) and folinic acid (FA). Two human tumor cell lines were used; one line was derived from colon cancer (WIDR) and the other from a non-small cell lung cancer (CAL 12). Cytotoxic effects were assessed by the MTT semi-automated test in 96-well incubation plates. The effects of various drug combinations were evaluated by the isobologram method. The drug combinations tested included fotemustine concentrations of 20, 30, 40, 50 and 70 micrograms/ml, 5-FU concentrations of 5, 15 and 30 micrograms/ml, and a constant FA concentration of 10(-5) M. A total of 180 different experimental conditions were tested. When cells were exposed to fotemustine before 5-FU the final cytotoxic effects for both cell lines were additive or synergistic in the majority of cases (P less than 0.001). The 5-FU concentration was a determinant factor affecting modification of the effects of the drug combination from antagonism (with low 5-FU concentrations) to synergism (high 5-FU concentrations) (P less than 0.001). Addition of FA (10(-5) M) resulted in a significant shift towards synergistic associations for both cell lines. Administration of 5-FU before fotemustine caused a marked antagonism which 10(-5) M FA was unable to significantly shift towards simple additivity.