In the present study we have demonstrated that orthovanadate at concentrations of 5-10 uM is cytotoxic to proliferating cells including primary cultures and tumour cell lines. However, concentrations of up to 50 uM did not affect the viability of non-proliferating cells. The cytotoxicity appears to be dependent on the vanadium concentration rather than on the oxidation state of vanadium or the vanadium compound. Furthermore, tumour cell lines with different proliferative rates were equally sensitive to orthovanadate cytotoxicity. Although the mechanisms responsible for the cytotoxicity are not known, addition of H2O2 potentiated orthovanadate cytotoxicity suggesting that hydroxyl or vanadium radicals may be involved. In vivo subcutaneous injections of orthovanadate into mice containing MDAY-D2 tumours resulted in the inhibition of tumour growth by 85-100%. These data indicated that orthovanadate at concentrations greater than 5 uM has antineoplastic properties and may be useful as a chemotherapeutic agent.