We conjugated tumor necrosis factor-α (TNF-α) with the synthetic polymeric modifier polyvinyl pyrrolidone (PVP) to facilitate its clinical use for anti-tumor therapy. TNF-α was chemically conjugated with the terminal carboxyl-bearing PVP at one end of its main chain, which was radically polymerized via the formation of an amide bond between the lysine amino groups of TNF-α and carboxyl group of PVP.In vitrospecific bioactivity of PVP-conjugated TNF-α (PVP-TNF-α) relative to that of native TNF-α gradually decreased with increases in the degree of PVP attachment. In contrast, PVP-TNF-α in which 40% of TNF-α lysine residues were coupled with PVP (MPVP-TNF-α) exhibited the highest anti-tumor activity among the conjugated derivatives examined. MPVP-TNF-α had more than 200-fold higher anti-tumor efficacy than native TNF-α, and the anti-tumor activity of MPVP- TNF-α was more than 5-fold stronger than that MPEG- TNF-α which had the highest anti-tumor activity among PEG-conjugated TNF-αs examined. Additionally, a high dose of native TNF-α induced toxic side-effects such as body weight reduction, piloerection and tissue inflammation, while no side effects were observed following i.v. administration of MPVP-TNF-α. The plasma half-life of MPVP-TNF-α (360 min) was about 80 and 3-fold longer than those of native TNF-α (4.6 min) and MPEG-TNF-α (122 min), respectively. These results suggested that PVP is a useful polymeric modifier for increasing the anti-tumor activity of PVP.