A hapten is obtained by replacing the 3-hydroxy group of 17 .alpha.-dihydroequilin with HO--CO--A--O-- wherein A is an alkylene of one to six carbon atoms. The hapten is conjugated with an immunological carrier to provide an immunogen, which in turn produces a specific antiserum to 17 .alpha.-dihydroequilin. The antiserum is used in a radioimmunoassay for 17 .alpha.-dihydroequilin.