The effect of stress on the immune capabilities of mammalian hosts has been recognized for many years. The hormone-mediated effect in this complex self-regulatory system are difficult studies involving many scientific disciplines. Only recently have scientists working in the new fields of psychoimmunology and neuroimmunology produced some quantitative data en the effects of stress on the cellular immune system in humans. These data will be reviewed and discussed in relation with feasible approaches that can be taken with marine mammals.