The widespread use of enzymes as drugs or therapeutic agents has been limited by (a) enzyme availability, (b) biodegradation of administered enzyme, (c) immunogenecity of the enzyme as a foreign protein, and (d) accessibility of the enzyme to the appropriate site of action. It has become obvious that due to these limitations, the administration of free or native enzyme is not likely to be effective. Various mechanisms of protecting or packaging enzymes to offset some of these drawbacks have been described. We have been successful in producing conjugates of a number of different enzymes with a molar excess of homologous albumin. The resulting enzyme-albumin complex is resistant to proteolytic and heat inactivation and is apparently non-immunogenic. Using specific ligands crosslinked to the enzyme-albumin conjugate we have been able to target these conjugates to specific receptor sites and specific tissues. Ligands including cell surface-recognizing antibodies and hormones such as insulin have been used. These approaches offer new possibilities for the increased use of enzymes in medicine.