Hormone-sensitive lipase (HSL) is a multi-functional enzyme involved in several aspects of lipid metabolism. Limited tryptic digestion of HSL led to selective loss of activity against lipid substrates but not against the water-soluble substrate, p-nitrophenyl butyrate. Following labelling of the active site of HSL with either [3H]di-isopropylfluorophosphate or [14C]orlistat, tryptic digestion of HSL generated a stable radiolabelled domain of molecular mass approx. 17.6 kDa, consistent with this representing a catalytic domain of HSL capable of hydrolysing water-soluble but not lipid substrates. Following phosphorylation of HSL by cyclic AMP-dependent protein kinase, limited tryptic digestion produced a stable phosphorylated domain of molecular mass 11.5 kDa. Based on these experimental data a model for a domain structure of HSL is proposed.