We report the preparation and application of polyclonal antisera to the analysis and quantitation of human interleukin-1 alpha (IL-1 alpha) and human interleukin-1 beta (IL-1 beta). The anti IL-1 alpha antibodies specifically react with the alpha form of IL-1 and do not cross react (less than 0.1%) with the beta form of IL-1 and vice versa. Data reported here demonstrate that detection of human IL-1 alpha or beta by a radioimmunoassay technique is sensitive enough to measure picogram levels of these lymphokines. The practical application of using these highly specific antisera for radioimmunoassays was established by measuring exogenously added IL-1 alpha or IL-1 beta to human plasma. Potential benefits of these reagents and the radioimmunoassay procedures described herein are discussed in relation to the biological assays which cannot distinguish between human IL-1 alpha and human IL-1 beta.