Monospecific antisera against three glutamate dehydrogenase (GDH) subunits of lupin root nodules were obtained. The use of sensitive mixed rocket immunoelectrophoresis enabled detection of seven GDH forms at the early stage of nodule development, thus providing evidence for the earlier hypothesized (L. Ratajczak et al., 1986, Physiol. Plant., 67, 685-689) random association of subunits 2g and 2a to form the remaining five GDH forms. All seven forms were localized in mitochondria. Immunological similarity was found between form 1 and plastid GDH.