Rats are immunised with synthetic oligopeptide (containing an N-terminal cysteine)-carrier protein conjugates. IL-10 was conjugated to JES3. The mouse non-secreting myeloma fusion partner P3X63-AG8.653, is fused to immunised rat splenocytes to produce heterohybridomas secreting rat antibody. Bioassays were used to identify neutralising mAbs, and immunochemical screens were used to identify mAbs that bind the cytokine.