In view of the clinical potential of a number of natural products, Combretastatin A-1 phosphate was developed as a water-soluble derivative of combretastatin A-1. This study examined the anti-tumour activity of this compound against an experimental colon tumour (MAC29) in mice. A comparison was made with the clinically active combretastatin A-4 phosphate. The new compound was well-tolerated up to a dose of 250 mg/kg and was more effective at producing tumour growth delays than the A-4 analogue. Significant growth delays were seen at a dose of 50 mg/kg whereas the A-4 phosphate produced no measurable growth delay until a dose of 150 mg/kg was administered. Histological examination of treated tumours indicated that combretastatin A-1 phosphate caused very severe haemorrhagic necrosis in the tumour tissue and analysis of the sections indicated that almost 94 percent of the tumour was dead within 24 hours of treatment. The mechanism of action of combretastatin A-1 phosphate appears to be similar to the A-4 phosphate in that tumour vascular shutdown occurs within 4 hours of treatment. In summary combretastatin A-1 phosphate, the water-soluble analogue of combretastatin A-1, is more potent against a well-vascularised murine colon tumour than its predecessor, combretastatin A-4 phosphate. These data suggest this compound may have potential for clinical development.