The interactions of surfactants with the enzyme Rhizomucor miehei lipase have been studied. From earlier studies, it is known that the enzyme binds to cationic surfactants but not to nonionic and anionic surfactants. The influence of the charge of the surfactant has been studied using surface tension, ellipsometry and micro calorimetry techniques. The study has included several anionic, nonionic and cationic surfactants, but the major part of teh work has been concerned with a zwitterionic surfactant (N-(2-hydroxy) dodecyl sarcosine sodium salt) The zwitterionic surfactant changes its charge from anionic at high pH to cationic at low pH values. It has been confirmed that when the headgroup is positively charged, the surfactant binds to teh enzyme. However, the number of surfactants that bind to the enzyme is dependent on the structure of the surfactant. An interaction model has been proposed. Negatively charged sites on the enzyme are available for binding cationic surfactant headgroups. Furthermore, a hydrophobic group is available close to the anionic site, which makes a hydrophobic attraction possible between the tail of the cationic surfactant and the enzyme.