BACKGROUND: The study aimed at evaluating the potential benefit from a combination of fractionated ionising radiation with the vascular-targeting compound combretastatin A-4 phosphate (CA-4-P). MATERIALS AND METHODS: Syngenic rat rhabdomyosarcoma (R1), growing subcutaneously, was treated at 2 different sizes: either small (2 +/- 0.5 cm3) or large (10.94 +/- 0.6 cm3). Localised fractionated irradiation of the tumours (5 x 3 Gy) in 5 days was followed 1 day later by an intraperitoneal CA-4-P treatment (25 mglkg). RESULTS: The combined treatment of only large tumours resulted in a small additional growth delay when compared with radiotherapy only. CONCLUSION: The present data indicate a size-dependent increase in tumour growth delay from combining fractionated irradiation with CA-4-P.