Incubation of human peripheral blood monocytes (PBM) with a partially purified thymic preparation, thymosin fraction (TF5), results in a dose dependent production of an interleukin-1 (IL-1)-like factor. The biological activity of this factor can be blocked by anti-IL-1 alpha, but not by anti-IL-1 beta which neutralizes bacterial induced IL-1 activity. Studies with further purified TF5 fractions show that this activity is not due to the well-characterized peptide, thymosin alpha 1 (T alpha 1), but rather a new thymosin peptide(s) isolated from a more basic fraction. Intraperitoneal injection of TF5 also induces the expression of a membrane-bound IL-1 (mIL-1) on mouse peritoneal cells. This study provides the first evidence that TF5 can influence macrophage activity directly by enhancing IL-1 production. This observation may help explain the mechanism by which TF5 modulates immune responses. These results also point to a more selective role for thymic hormones, growth factors and cytokines which may trigger macrophages to secrete different forms of IL-1, which can then regulate either immune and/or inflammatory processes.