To help dissect the molecular basis of the Rhizobium-legume symbiosis, we used in vitro translation and Northern blot analysis of nodule RNA to examine alfalfa-specific genes (nodulins) expressed in two types of developmentally defective root nodules elicited by Rhizobium meliloti. Fix- nodules were elicited by R. meliloti nif mutants; these nodules were invaded by rhizobia and contained differentiated bacteroids. 'Empty' nodules were elicited by R. meliloti exo and ndv mutants and by Agrobacterium tumefaciens strains carrying the R. meliloti nod genes; these nodules contained a nodule meristem but lacked infection threads, intracellular bacteria, and bacteroids. Fix- nodules contained a spectrum of nodulins similar to wild-type nodules. In contrast, only two nodulins, Nms-30 and a nodulin homologous to ENOD2 of soybean, were detected in empty nodules. Although R. meliloti ndv and exo mutants elicited nodules with the same defective phenotype, ndv and exo mutants (except for exoC mutants) had distinct biochemical phenotypes. R. meliloti ndvA and ndvB mutants were deficient in cyclic glucan production but not the acidic exopolysaccharide; the converse was true for exoA, exoB, and exoF mutants. exoC mutants were defective in both exopolysaccharide and cyclic glucan biosynthesis. Our results support the model that the R. meliloti nod genes produce a signal that results in nodule meristem induction. Both the exopolysaccharide and cyclic glucan, however, appear to act at the next step in the developmental process and are involved in the production of a signal (or structure) that allows infection thread formation and invasion of the nodule.