Human peripheral blood monocytes stimulated with lipopolysaccharide (LPS) produced two major proteins which enhanced the proliferative response of thymocytes to mitogenic stimulation (IL-1 activity). These proteins were of the same molecular weight (about 14,000) but had different ionization properties (pI 5.1 and 6.8). Treatment with neuraminidase did not alter the pI or the activity of either species, indicating that they did not differ solely in their extent of sialyation. Tunicamycin, at levels which markedly inhibited the incorporation of carbohydrate into proteins secreted by LPS-stimulated macrophages, failed to affect the two forms of IL-1. IL-1 also did not bind to concanavalin A-Sepharose. Thus, carbohydrate groups did not appear to play an important role in the structure or activity of the two species of IL-1. However, the biological activity of these two species of IL-1 was dissociated by treatment with N-ethylmaleimide (NEM), iodoacetamide (IAA), and heat. The protein with a pI of 6.8 was significantly inactivated by NEM, IAA, and heat, whereas the IL-1 with a pI of 5.1 was minimally affected. It appears that the two species of IL-1 differ in the availability of reactive groups, which are most probably sulfhydryls, and, consequently, also in tertiary structure.