Chimpanzees, because of their near-human nature, have special needs that must be met by those who carry out medical research with them. Perhaps the most important of these is the need for companionship. Chimpanzees kept alone become obviously depressed, and manifest stereotypic behavior such as compulsive rocking. We have followed a policy of keeping chimpanzees in groups of two or more, whenever possible. This has virtually prevented overt depressive symptomatology. This policy has not significantly affected studies on hepatitis B and C (1).