Two mouse monoclonal antibodies (Mabs) against recombinant human interleukin-5(rhIL-5) have been produced, characterised and purified. Both are IgG1 antibodies and neutralised the activity of rhIL-5 in the B13 assay. Neither Mab cross-reacted with mouse IL-5. A two-site sandwich enzyme-linked immunosorbent assay (ELISA) was developed with different combinations of the mouse Mabs and also a rat anti-mouse IL-5 Mab, TRFK5, which also has activity against rhIL-5. The most sensitive assay, with a lower detection limit of 0.5 ng/ml IL-5, used TRFK5 as the capture antibody and the mouse anti-human IL-5 Mab as second antibody. The sensitivity of this assay was increased by an enhanced chemiluminescent reagent and resulted in a lower limit of detection around 40 pg/ml IL-5.