Effects of NaCl (50, 100 and 150 mM), proline (2.5, 5 and 10 mM), and their combinations on growth and contents of chlorophyll, proline, soluble protein and polyphenol oxidase (PPO E.C. 188.8.131.52) activity of maize (Zea mays cv. Merit) seedlings cultured in vitro were investigated. NaCl decreased seedling growth, fresh weight and chlorophyll content, while soluble protein and proline contents were increased. Proline added to control seedlings decreased root length and fresh weight, but did not change shoot length. Also proline increased chlorophyll, soluble protein and proline contents. When proline was added into the medium of NaCl-treated seedlings, growth, chlorophyll, soluble protein and proline contents were increased, in comparison to seedlings treated only with NaCl. PPO activity was increased by 5 mM proline treatment in all tissues of the maize seedlings growing in the salt medium. The results suggest that proline may play a role as an enzyme protector under conditions of salt stress.