Interleukin-1, first identified as a macrophage factor of importance in infections and inflammation, is a protein with properties of an endogenous pyrogene and lymphocyte activating factor. Occurrence of interleukin-1-like immunoreactivity was demonstrated in the noradrenergic chromaffin cells of the rat and mouse adrenal gland by means of two antisera raised against synthetic peptides corresponding to the amino acid sequences 169-194 and 201-215 of the murine interleukin-1 precursor protein. These antisera also inhibited stimulation of interleukin-2 receptor expression by purified human interleukin-1. Reserpine, which is known to deplete catecholamines, also caused release of the interleukin-1-like immunoreactive material. The interleukin-1 content of the rat adrenal medulla was estimated by radioimmunoassay, and if released the adrenal interleukin-1 pool may result in plasma interleukin-1 levels of about 10(-12). The interleukin-1-immunoreactive material obtained from the rat adrenal gland was characterized as a trypsin-sensitive protein with a molecular weight in the range of 13,000-19,000. This protein fraction stimulated interleukin-2 receptor expression on human T-cells as earlier shown for interleukin-1.