The aim of this study was to evaluate the effect of Ampicillin as bacteriostats on the number of P solubilizing and antibiotic-anti- Rhizobium. meliloti cells and contamination in inoculants during one year storage, and plant promotion ability of subgeneration cells of antibiotic-resistant phosphate-solubilizing R. meliloti strain LW107 growth on YMA plates containing different concentrations of ampicillin were also investigated. In the first experiment, 100 mg liter-1 ampicillin was added in inoculants, number of R. meliloti cells and contamination in inoculants were investigated at two temperatures, using both liquid and peat based solid inoculants in experiment. Statistical design was a complete randomized block in a factorial 2×2×2 experimental arrangement with 4 replicates. Results show that there are various viable cells and contamination levels in inoculants. The ampicillin increased R. meliloti LW107 viable cells and inoculants stored at low temperature with lesser contamination when ampicillin been added. To clarify whether ampicillin affect the main character of R. meliloti LW107 cells, the ability of calcium phosphate solubilization and IAA production in ampicillin-containing conditions was determined. Results indicated that the ability of calcium phosphate solubilization and IAA production have no significant difference on 6th generation cells of R. meliloti LW107 between the growth plates and the lack of ampicillin. In the third experiment, the plant growth promotion of the ampicillin-containing inoculants on alfalfa seedlings was determined in sterile sand conditions in a temperature-controlled growth chamber at 20 -25°C for 35 days. We also found that no significant difference on nodule numbers between diluted liquids ampicillin-containing inoculants and the common liquid inoculants containing no ampicillin, but plant growth and efficiency of nitrogen-fixing responded to undiluted liquids inoculants containing ampicillin with a decrease in biomass and nitrogenase activity indicating that the five-fold dilutions of the ampicillin-containing inoculants are necessary for applications.