An implantable medical device has a medical unit, such as a pacemaker lead, and a casing at least partially enclosing the medical unit. The casing is formed of a base polymer such as a polyurethane, a polyurethane copolymer, a fluoropolymer and a polyolefin or a silicone rubber. The casing has biologically active agents covalently bonded to the base polymer. The biologically active agents can be attached to the base polymer as surface active end groups. As an alternative, the biologically active may be attached to a backbone the base polymer. As yet a further alternative, the biologically active agents may be attached to surface modifying end groups, which are in turn attached to the base polymer. Examples of suitable biologically active agents are microbial peptide agents, detergents, non-steroidal anti-inflammatory drugs, cations, amine-containing organosilicones, diphosphonates, fatty acids and fatty acid salts.