The nod C gene of Rhizobium meliloti encodes a protein of mol. wt. 44 000 which is highly conserved in at least three Rhizobium species. In order to overproduce this protein, a gene fusion of lambda cI repressor sequences to a large fragment of nod C was constructed. The fusion was placed under control of the tac promoter on plasmid pEA305 to yield pJS1035. IPTG-induced Escherichia coli cells harbouring pJS1035 accumulated the cI-nod C hybrid protein up to 19% of total cellular protein. The synthesis of the hybrid protein drastically inhibits the growth rate of the bacterium. The fusion protein was purified by gel and hydroxyapatite chromatography in the presence of SDS. Antibodies raised against the purified fusion protein precipitated the mol. wt. 44 000 nod C proteins of R. meliloti and of the broad-host range Rhizobium strain NGR234, which were both expressed in E. coli mini-cells. The hybrid protein is associated with the outer membrane of E. coli cells, and the cI-nod C fusion protein appears to be an integral membrane protein. Nodulation of alfalfa by R. meliloti and of clover by R. trifolii was markedly inhibited (approximately 50%) by the addition of antibodies against the hybrid protein to plant growth medium and inoculum.