The purpose of this study was to evaluate the effect of ampicillin as bacteriostats on the doubling time and relative survival rate of antibiotic resistant Rhizobium strains and the ratio of undesirable microbes in artificially polluted inoculants during 60 days storage, and the nodule occupancy of two bacteriostat-resistant strains were also investigated. Fully grown Yeast-mannitol liquid media culture of Rhizobium meliloti LW107 and Rhizobium sp. RSW 96 were added with ampicillin as bacteriostats at various concentrations in an artificial pollution test, and the relative survival rate of rhizobia and the counts of undesirable microbes were determined by the agar plate dilution method. The result shows that in the artificially polluted liquid inoculants, the survival rate of rhizobia was increased and the ratio of undesirable microbes declined significantly when the ampicillin was used as the bacteriostats. The growth of selected antibiotic-resistant strains was promoted at the concentration of less than 100 ug/ml and the doubling time was reduced, but inhibited and the doubling time was significantly prolonged at the concentration of more than 200 ug/ml. Inoculation tests also shows an increased nodulation competitiveness of two bacteriostat-resistant strains in five-fold dilutions of liquid inoculants containing ampicillin (at optimal concentration). According to comprehensive consideration, the optimum concentration for ampicillin as bacteriostat in Rhizobium meliloti LW107 and Rhizobium sp. RSW 96 inoculants appeared to be 100 ug/ml and 200 ug/ml, respectivly.