The interaction of radiation (10 Gy 300-kV X-rays) and methotrexate (MTX; 3 x 10 mg/kg at 3.5-h intervals) was investigated with respect to effects on cell survival and tumour regrowth of the transplantable rat R-1 rhabdomyosarcoma. The treatment with MTX alone caused acceptable toxicity and no lethality. On day 3 after treatment with MTX alone a maximum decrease in the fraction of clonogenic cells was observed, which is in accordance with data on MTX concentrations in tumour tissue, indicating that MTX is active in the tumour for at least 3 days after injection. The clonogenic capacity after combined treatments, i.e. MTX before or after radiation, was assessed 3 days after the MTX administration. The fractions of clonogenic cells determined after combined therapy with intervals of up to 4 days were not significantly different from those expected on the basis of simple addition of the effects from individual treatments. However, the excess growth delay was positive at specific intervals (6-8 days after X-rays plus MTX and 5-6 days after MTX plus X-rays), whereas negative excess delays were observed when the two treatments were separated by less than 3 days. It is concluded that expectations with respect to clinical application of the combination must be modest in view of the short duration of favourable intervals and the observed absence of synergistic effects with respect to cell killing. The discrepancy between the two assays indicates that erroneous conclusions can be obtained if one endpoint only is assessed.