Abstract A sensitive immunoassay procedure for the detection of the thrombopoietic-stimulating factor (TSF) has been developed. This procedure is similar to the hemagglutination-inhibition (HAI) technique that is available for erythropoietin. For the production of TSF, sheep were made thrombocytopenic by the injection of antiplatelet serum. TSF from sheep sera was partially purified by column chromatography and tested for biological activity in thrombocythemic mice. A TSF-rich serum fraction was used to immunize rabbits, and the immune serum was tested for hemagglutinating antibodies. The hemagglutinating antisera were absorbed with normal sheep serum to remove antibodies not specific to TSF. The absorbed antisera containing TSF-specific antibodies were then used to detect TSF in platelet-depleted sheep sera by use of the HAI assay procedure. The data indicate that TSF can be detected and quantified in sheep serum by use of this technique.