Cancer antigen 125 (CA 125) is a useful tumor marker with a threshold value of 35 U mL(-1) expressed by more than 80% of patients with non-mucinous epithelial ovarian cancer. In this work, a novel strategy for the analysis of CA 125 using the plasmon resonance scattering (PRS) properties of gold nanorods was proposed. The gold nanorod (GNR)-anti-CA 125 conjugates were prepared first. The immunoreaction between the GNR modified CA 125 antibody and CA 125 took place in aqueous solution after the addition of CA 125. The target protein concentration was determined by analyzing the level of GNR aggregation caused by CA 125 antibody-antigen interactions using PRS. At a concentration of 1.0-80 U mL(-1), CA 125 could be determined with a detection limit of 0.4 U mL(-1). The PRS immunosensor showed high sensitivity, selectivity, and reproducibility. Determination of CA 125 in serum samples showed good recovery.