A deletion derivative of the cytokine human interleukin-3 (hIL-3(15-125), comprising amino acids 15-125 of the native protein) was produced as a fusion to the filamentous phage surface protein pIII. The cytokine was detected in association with phage particles by protein immunoblotting. Compared to an equivalent quantity of soluble cytokine, phage-presented hIL-3(15-125) exhibited reduced biological activity in a hIL-3-dependent cell proliferation assay. The reduction in activity was attributable to presence of phage particles in the assay, rather than directly owing to physical incorporation of the cytokine into the phage particle. Owing to the position of the amber codon in the phagemid vector, the phagemid-produced free hIL-3(15-125) species (designated hIL-3(15-125) epsilon) had 20 amino acids appended to its C-terminus; hIL-3(15-125) epsilon did not exhibit reduced bioactivity. hIL-3(15-125)-presenting phage were affinity-selected with either a hIL-3-reactive polyclonal antibody or with cells expressing the heterodimeric hIL-3 receptor. These data are consistent with the use of phage-display technology for the affinity selection of hIL-3 variants with modified biological properties.