The physico-chemical stability and the compatibility between a 2.5-mg/ml methotrexate solution and plastic syringes were studied over a 7 day period at +4 and 25°C. During this study, pH was determined and MTX degradation products were assayed by HPLC. The compatibility study consisted of the titration of reducing substances, TLC of extracted additives, assay of extracted ions by AAS, resistivity measurement, dynamometric test and UV, visible and IR absorption spectrophotometry. Stability and compatibility studies showed that 2.5 mg/ml methotrexate parenteral solution remained chemically stable in a plastic syringe over a period of 7 days under the tested conditions.