An immunoassay for estrone (E-1) in women's serum, based on the competitive reaction between fluorescein-labeled complete antigen and E-1 with limited amount of anti-estrone monoclonal antibody is described. A thermally reversible hydrogel, poly-N-isopropylacrylamide (pNIPA), was added to the buffer to improve the reproducibility. With a laser-induced fluorescence (LIF) detector, the capillary electrophoretic immunoassay (CEIA) can be applied to determine E-1 at a concentration lower than 19.6 pg/mL. The E-1 levels in ten normal women's serum were measured at the range of 118.6-222.0 pg/mL.