Follicle regulatory protein (FRP) is secreted by the granulosa cell of the ovary and plays a role in modulating follicle development. A dual epitope immunoassay using two murine monoclonal antibodies, isotype IgG1 (raised against porcine FRP), in tandem was developed to measure FRP in serum. The levels of FRP in the serum of women with granulosa cell tumors, normal, menstruating women, and postmenopausal women were determined. The levels of FRP were elevated in the serum of 79% of the women with granulosa cell tumors compared to the normal controls. FRP levels in serial samples from women with granulosa cell tumors generally correlated with the clinical course of the disease. Thus, FRP may provide a useful marker for granulosa cell tumors.