The changes in the contents of protein and free amino acids in pea plants inoculated with Rhizobium leguminosarum were studied taking into account the susceptibility of roots to root nodule bacteria. The content of cytoplasmic protein during infection increased in the actively growing root region (0-5 mm) and decreased in the root regions susceptible to rhizobia (5-20 mm from the root tip). The quantitative composition of free amino acids changed essentially upon inoculation of pea seedlings with R. leguminosarum.