Some growth factors are therapeutically useful partly because restricted expression of their receptors limits their action to particular cell types. However, no unique stimulatory factor is known for many clinically relevant cell types, such as CD34(+) hematopoietic stem cells. Here, soluble alpha receptor (R alpha) components for interleukin-6 (IL-6) and ciliary neurotrophic factor (CNTF) were targeted in an active form to cells expressing surface markers such as CD34 or CD45, thereby rendering those cells responsive to IL-6 or CNTF. The targeting of R alpha components may provide the means to create "designer" cytokines that activate a desired cell type expressing a specific cell surface marker.