Active immunization against infectious disease is important. However, much of our world faces poverty, social injustice, and warfare, all of which cause universal immunization to remain a distant dream. Agents that provide passive immunity thus remain essential biologicals. The most important of these are human or equine antisera against rabies, tetanus, diphtheria, and snake antivenins. Homologous products are either unavailable or unaffordable in places where they are needed the most. Less expensive heterologous (equine) antisera can be purified and are safe to use, but these antisera are also in short supply. Monoclonal antibodies have been developed but are even less likely to be affordable in poor countries. Several traditional sources of equine antisera are becoming depleted as a result of economic disincentives; a poor reputation based on the high adverse reaction rates of the old, unpurified products; and the activities of animal rights activists who object to the use of horses as blood donors. Purified, pepsin-digested equine antisera are preferred; but developing countries sometimes are forced to make crude products that are less safe or have no specific therapy available at all.