The potentiating effects of a variety of recombinant cytokines on the survival or proliferation (or both) of sheep bone marrow eosinophils in vitro were assessed by means of cell-specific enzyme microassays for eosinophil peroxidase (EPO) and eosinophil arylsulphatase (EAS). In this system recombinant human and mouse interleukin 5 (rhIL5 and rmIL5, respectively) had potent eosinophil potentiating activity (EPA) that was reciprocally inhibited by polyclonal anti-rhIL5 and anti-rmIL5 antibodies and by a specific monoclonal anti-rmIL5 (TRFK5). Recombinant ovine granulocyte-macrophage colony-stimulating factor (rovGM-CSF) and interleukin 3 (rovIL3) also had marked EPA for sheep cells, and the activity of the former was blocked by a mouse monoclonal anti-rovGM-CSF. The equivalent human and murine recombinant GM-CSFs and IL3s had no detectable effect on sheep eosinophils, nor did antibodies against them influence the EPA of any of the ovine cytokines. The evidence presented provides further support to the concept that the structure, biological reactivity and cell specificity of IL5 is highly conserved in mammals, whereas other eosinopoietins such as IL3 and GM-CSF are more species-specific.