Nodule specific proteins (nodulins) were detected by the 2D-polyacrylamide gel electrophoresis of the immunoprecipitated in vitro translation products of the poly(A)^+-mRNA from alfalfa root nodules. At least 25 nodule specific polypeptides in the molecular weight range of 12,000-30,000 were identified in the Rhizobium meliloti 102F51 (wild type) infected alfalfa root nodules. The nodulins accounted for about 40% of [³H]-leucine labeled and 12% of [^(35)S]-methionine labeled proteins.