Chloroplastic glutamine synthetase from tobacco leaves (Nicotiana tabacum L. var Xanthi) was purified to homogeneity. By using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and high performance liquid chromatography, a single subunit was identified with a molecular weight of 45,000 daltons. However the native protein seems to be composed of four different subunits which can be separated by isoelectrofocusing. It is suggested that different genes with eventual posttranslational and/or posttranscriptional modifications may control the synthesis of the chloroplastic glutamine synthetase.