Several synthetic interleukin-1 (IL-1) peptides were tested in vivo for pyrogenic activity and in vivo for their ability to stimulate prostaglandin production. Only the IL-1β fragment (208-240) enhanced body temperature, although both IL-1β (208-240) and IL-1α (223-250) stimulated prostaglandin E 2 (PGE 2 ) production in vitro. We report here that the IL-1β fragment (208-240) did not have the capacity to induce arachidonic acid (AA) mobilization by 3T6 fibroblasts. However, this peptide was able to increase the expression of the inducible prostaglandin H synthase isoform (PGHS-2; EC 1.14.99.1.), which is related to its ability to stimulate prostaglandin E 2 synthesis.