Vanadium, as ammonium monovanadate, has been found to stimulate tumour cell proliferation in mice bearing a transplantable ascitic lymphoma. Markers including microsomal cytochrome P-450, UDP-glucuronyltransferase and cytosolic glutathione-S-transferase showed substantial alterations in a dose-responsive manner with vanadium administration when compared to the controls. Stimulation of tumour progression is also reflected by increased tumour cell count and decreased survival of the host.