On M2-medium, which is a revised Linsmaier and Skoogmedium, callus tissue of Vitis vinifera cv. Müller-Thurgau shows normal growth not dependent upon myo-inositol. Callus cells of this cultivar may therefore be able to synthesize myo-inositol. Callus cells of the cv. Aris require myo-inositol and show increasing growth up to an optimum of 250 mg 1(-1). Further increase of the myo-inositol concentration resulted in decreasing growth. Growth promotion may be due to a higher cell division rate and not to cell enlargement. On a medium supplemented with 350 mg KH(2)PO(4)l(-1) callus tissue growth of cv. Aris increased with increasing concentration of myo-inositol up to 4000 mg 1(-1). The positive effect may be explained by the possible mode of action of myo-inositol, which, incorporated into phosphatidylinositol, may be an important factor in the functioning of membranes. The myo-inositol dependence of cv. Aris callus tissue cells may be traced back to its hybrid origin in which Vitis riparia is one of the ancestors. Callus of V. riparia and V. rupestris is as dependent upon myo-inositol as cv. Aris.