High expression of folate receptors is characteristic for the effector cell population of synovial macrophages in synovial inflammation. A new drug conjugate, EC0746, targets the folate inhibitor aminopterin to folate receptors. In vitro studies show that this conjugate acts antiproliferatively and inhibits cytokine production by macrophage cell lines. Moreover, it shows strong anti-arthritic effects in the rat model of adjuvant-induced arthritis in vivo. Toxicity of aminopterin was reduced 40-fold by using equimolar doses of the drug conjugate. In conclusion, this new treatment approach has the potential to further improve the most successful principle of folate inhibition in the treatment of arthritis.