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The present article describes procedures to measure rat IL-4 protein. The RT-PCR technique has been successfully and widely used to measure IL-4 mRNA, but it does not determine IL-4 protein synthesis. Assays to measure rat IL-4 protein based on its biological activity were developed using the mAb OX-81, which inhibits rat IL-4 activity. Two bioassays were attempted based on the ability of IL-4 to induce the proliferation of T cell blasts and to increase MHC class II expression on resting B cells. A second mAb against rat IL-4 was used in a sandwich ELISA to detect rat IL-4. This ELISA is satisfactory although its sensitivity is not as high as that of the bioassay. According to our experience, the bioassay based on the induction of class II MHC molecules on B cells is the technique of choice for rat IL-4 determination because it proved specific, sensitive and reproducible.