Microcalorimetry has been used to study the interaction be tween Humicola lanuginosa lipase and either the anionic surfactant sodium dodecyl sulfate, SDS, or the cationic surfactant tetradecyltrimethylammonium bromide, TTAB. For SDS no conclusive evidence was obtained of whether or not an enzyme surfactant complex is formed. Although not unambiguous, the calorimetric titration curves obtained with TTAB indicate formation of such a complex.