Purpose. Results of an assessment of the chemical stability of isoniazid injection in 0.9% sodium chloride injection and 5% dextrose injection are reported. Methods. Triplicate solutions of isoniazid (0.5 and 6.0 mg/mL) in the 2 diluents were prepared in ethylene and propylene copolymer i.v. containers and stored under light protection at room temperature (20–25 °C) or under refrigeration (2–8 °C). Standard aliquots were removed from each solution at time points up to 72 hours and analyzed via high-performance liquid chromatography (HPLC). Stability was defined as retention of >90% of the initial isoniazid concentration; pH, osmolality, and visual appearance were assessed. Results. Isoniazid 0.5- and 6.0-mg/mL solutions in 0.9% sodium chloride injection were stable for up to 72 hours at room temperature or under refrigeration. HPLC analysis of isoniazid 0.5-mg/mL solutions in 5% dextrose injection revealed a decrease to less than 90% of the initial concentration at 8 hours at room temperature and at 30 hours under refrigeration. Isoniazid 6.0-mg/mL solutions in 5% dextrose injection were stable for 24 hours at room temperature and for 48 hours under refrigeration. The pH, osmolality, and visual appearance of the solutions were not affected. Conclusion. Isoniazid solutions of 0.5 and 6.0 mg/mL in 0.9% sodium chloride injection were stable under light protection for up to 72 hours when stored at room temperature or under refrigeration. Isoniazid injection was less stable in 5% dextrose injection, especially at a concentration of 0.5 mg/mL at room temperature.