The loss of K+ from excised barley, wheat, chick pea and bean roots was measured in the absence or presence of different concentrations of NaCl and mannitol. In the presence of 50 mM Ca2+, the threshhold NaCl concentration for induction of K+ loss was 160 mM for barley, wheat and chick pea and 140 mm for bean. Mannitol iso-osmotic to 160 mM NaCl did not induce K+ loss from barley, wheat or chick pea roots indicating that the induction of K+ loss was a salt-induced and not an osmotic injury. The preventive effect of various concentrations of CaCl2 and MgCl2 on salt-induced K+loss indicated that the protective effect of Ca2+was specific. Although 50 mM Ca8+ was sufficient, concentrations up to 40 mM may be more effective. The measurement of salt-induced K+ loss appears to be a quick and reliable method of estimating the degree of salt resistance in plants.