Summary A mixed culture system of Rhizobium and legume cells was established in order to determine if any of the proliferative events of legume root nodulation could be reconstructed. Root callus cells of soybean (var. Acme), growing slowly in nitrogen-deficient Linsmaier-Skoog medium, were inoculated with cells of R. japonicum (USDA strain 311b138) in order to look for DNA synthesis and mitotic stimulation; DNA synthesis was monitored by 3H-thymidine incorporation and cell labeling in autoradiographs. A small initial increase in the frequency of labeled soybean cell nuclei (∼ 1.5 X controls) was detected, but this did not lead to mitotic stimulation because the soybean cells rapidly lost viability. The physiological basis for the stimulation, as well as its possible relationship to the proliferation of root cortex cells in the legume nodule, is discussed.