Using tobacco callus tissues the authors studied the antagonism of magnesium and aluminium, and the inhibition of aluminium toxicity by magnesium and citric acid, as expressed by their effect on growth and alkalod content. In response to an increase in the concentration of MgSO4 in the culture medium the fresh weight of the cultures significantly increased, while with an increase in Al2(SO4)3 growth decreased, 2 mM causing a high degree of inhibition. The toxic effect of aluminium was less marked when Mg was present at a concentration higher by an order (antagonistic inhibition). When citric acid was added to the culture medium, promoting complex formation with Al, the toxic effect of Al was not apparent. The nicotine content of the tissues was in each case inversely correlated with the magnitude of biomass formation.