Curcumin (Cur), the golden yellow phenolic compound in turmeric, is well studied for its medicinal properties. In the current investigation, Cur dissolved using sodium hydroxide solution (CurNa) was tested for in vitro complement inhibitory activity and compared with rosmarinic acid (RA) and quercetin (Qur) dissolved using sodium hydroxide (RANa and QurNa, respectively) and the vaccinia virus complement control protein (VCP). The comparative study indicated that CurNa inhibited the classical complement pathway dose dependently (IC50 = 404 microM). CurNa was more active than RANa, but less active than QurNa. VCP was about 2,212, 2,786, and 4,520 times more active than QurNa, CurNa, and RANa, respectively. Further study revealed that CurNa dose dependently inhibited zymosan-induced activation of the alternate pathway of complement activation.