Several carrier systems and targeting agents have been considered as means of delivering enzymes and drugs to specific tissues or cells. In this report insulin is shown to be effective in delivering enzyme-albumin conjugates to cells and tissues rich in insulin receptors. The complex is transported into cells by a process that resembles receptor-mediated endocytosis and can be identified in a lysosomal fraction. The enzyme-albumin-insulin complex retains its enzymatic activity and its ability to bind antibodies to insulin. It also has a hypoglycemic effect; however, plasma glucose concentrations can be maintained by glucose administration.