Although it is widely recognized that many proteins contain discrete functional domains, it is less certain whether smaller, less obviously discrete, units of structure will retain their specific function when transplanted into a different context. The observation that the potent inflammatory cytokine human interleukin 1 beta has the same overall structure as soybean trypsin inhibitor (STI) (Kunitz) prompted us to replace a tight turn in the cytokine sequence with the large loop in soybean trypsin inhibitor that binds to the active site of trypsin. Wild-type interleukin 1 beta (IL-1 beta) is highly resistant to proteolysis, but the chimeric STI/IL is specifically cleaved by trypsin, apparently in the inserted loop. Other chimeric interleukins have also been constructed, by replacing the same tight turn with inhibitory loops from other protein protease inhibitors: turkey ovomucoid inhibitor (TOI), a chymotrypsin inhibitor, and alpha 1-antitrypsin (AT), an elastase inhibitor. Although these loops come from proteins not related structurally to interleukin 1, they confer specific protease sensitivity or inhibition on the chimeric cytokine. The cytokine properties of these chimeric interleukins have also been evaluated. The chimeras formed from human IL-1 beta and all inhibitory loops tested bind to the interleukin 1 receptor with reasonable affinity. The typical cellular effects of IL-1, however, are not observed with all the recombinant proteins, thus confirming that receptor binding and signal transduction can be uncoupled. When these results are taken together with the results of site-directed mutagenesis of IL-1, reported in this paper and elsewhere, they allow the receptor and intracellular transduction sites on the protein to be mapped in detail.