Three different techniques of lipase (EC 3.1.1.3) determination (titrimetry, nephelometry, and enzyme immunoassay) were used to investigate an interesting effect of ionic strength on enzyme activity. Both activation in the presence of NaCl (140 mmol/L) and strong immunoinhibition were observed for lipase from plasma of subjects with acute pancreatitis. Another type of lipase, not associated with pancreatitis and only weakly immunoinhibited, showed maximum activity in the presence of 80 mmol/L NaCl. The pancreatitis-type lipase was activated by 140 mmol/L NaCl solution only if colipase and sodium deoxycholate were also present, which suggests that these components are cofactors in the activating effect and confirms the specificity of this property. These findings can be exploited to improve the diagnosis of acute pancreatitis.