Immunoprecipitation and two-dimensional gel electrophoresis analysis of the glutamine synthetase (GS) polypeptides (alpha and beta) during Phaseolus vulgaris root development shows that the alpha polypeptide is the main component of the enzyme in the embryo and in up to 5 day old roots. From 5 days on, the beta polypeptide becomes the root predominant GS monomer. The alpha/beta ratio of the in vitro translated GS polypeptides from the total polysomal RNA isolated at different root ages correlates with the alpha/beta ratio observed in the root extracts. These results suggest that the two root GS polypeptides are encoded by different mRNA species in Phaseolus vulgaris.