The effect of salt stress (NaCl and/or KCl) on the specific activity of superoxide dismutase (SOD) isozymes in leaves from cowpea plants and in mesophyll protoplasts from cowpea leaves was examined. In leaves, mitochondrial Mn-SOD was significantly decreased by 35 mM (up to 35%) and 100 mM NaCl (up to 60%), whereas, under the same conditions, cytosolic and mitochondrial Cu, Zn-SOD I was slightly diminished and chloroplastic Cu, Zn-SOD II did not show a response. In protoplasts, the specific activity of SOD isozymes was decreased as a function of both the salt concentration and the type of salt used Mn-SOD was again the most salt-sensitive isozyme, but in contrast to whole leaf preparations. Cu, Zn-SOD II in protoplasts was markedly inhibited by NaCl and Cu, Zn-SOD I showed an intermediate sensitivity. KCl concentrations equivalent to those of NaCl were less effective in inhibiting the SOD isozymes, suggesting the existence of specificity for certain ions. In protoplasts salt inhibition of SODs was reversible. However, Mn2+ was necessary for total recovery of Mn-SOD activity in NaCl-treated protoplasts. The in vitro results suggested a competitive inhibition of SOD isozymes by salts. A possible competitive inhibition of SOD activity in salt-treated cowpea plants was also suggested. Finally, the possible correlation between plant response to NaCl and the potential of SOD isozymes was analyzed.