We describe a sensitive ELISA for the measurement, of the recombinant human cytokine, interleukin-3 (IL-3), in human plasma or serum samples. The assay design uses two different anti-IL-3 monoclonal antibodies, giving a two-site assay configuration. The assay incorporates the use of alkaline phosphatase conjugated to streptavidin and a biotinylated anti-IL-3 monoclonal antibody to amplify the resultant signal. This ELISA can measure both glycosylated and non-glycosylated IL-3. The limits of quantification, as determined by precision profiles and quality control samples prepared in 100% human plasma, are 20 pg/ml and 30 pg/ml for non-glycosylated and glycosylated IL-3, respectively.