Evidence has accumulated indicating that four T-cell derived lymphokines, interleukin-4, interleukin-5, granulocyte-macrophage colony stimulating factor and interleukin-3, are closely related. The genes encoding these lymphokines are clustered within a region of 1000 kb, the proteins have a common secondary and spatial organization, and their corresponding receptors are all members of the haemopoietin family of receptors.