Application of sodium chloride to crop plants in the culture medium leads to an increase in their endogenous content of free proline. There is a certain concentration of sodium chloride above which the proline content of the plants strongly rises (critical point). A relationship between endogenous sodium and proline contents could be found. In salt-sensitive plants (wheat) the critical point lies below that of salt-tolerant plants (barley). The critical concentration is not changed by certain alterations of the culture medium. The determination of the critical point by means of measurement of the proline concentration served as a basis for the analysis of the salt tolerance of crop plants.