Proteins, such as hormones, enzymes, or antibody binding sites, can be expressed in an active conformation on the surface of filamentous bacteriophage. Although the phage display technology was originally developed for binding studies, we demonstrate here that this technique can rapidly provide cytokines for studies of biological activity and for raising neutralizing monoclonal antibodies. A phage M13-based cloning vector was constructed that facilitated the expression of human interleukin 3 (hIL-3) on the phage surface. The recombinant phage could stimulate the growth of the hIL-3 dependent cell line M-07, providing evidence for the display of hIL-3 in an active form. Injection of recombinant phage into mice provoked an immune response to hIL-3, and neutralizing monoclonal antibodies directed against native hIL-3 could be established from these mice with a high frequency.