Chemokine receptors play a major role in the mobilization and activation of the cells of the immune system, and are discussed in this review by Richard Horuk. Six receptors that bind chemokines with high affinity have been cloned and all belong to the superfamily of G protein-coupled receptors. Chemokine receptors are mainly expressed in immune cells, which are their major target cells. However, two of the cloned receptors are expressed by viruses and may play a role in protecting the virus from immune surveillance. In addition, a novel chemokine receptor that is expressed in human erythrocytes not only binds CC and CXC chemokines with high affinity but is also a receptor for the malarial parasite Plasmodium vivax.