Abstract: The invention relates to specific binding members for estradiol and materials and methods relating thereto, in particular antibodies or binding domains thereof which have high affinities (low dissociation constants) for estradiol and low cross-reactivity for other steroids. The invention further provides means to make such binding members, assay methods for the detection and/or quantitation of estradiol, and nucleic acid encoding said binding members, which nucleic acid may be used for their production. Preferred binding members include those with CDR regions of the D12 antibody heavy and light chain domains (SEQ ID NO:2 and SEQ ID NO:12 respectively).