inverse of the concentration ofcompetitor divided by the inverse of the concentration of pregnanediol thatis required for 50 decreases in binding of the labelled ligand the antiserum was found to be highly specific for the 20 u03b1 reagent of this invention as well as for pregnanediol and its glucuronide the reactivity of the 20 u03b1 reagent being of the same order as shown by the tabulated results table a______________________________________specificity of anti pregnanediolglucuronide antiserumsteroid competition______________________________________5 u03b2 pregnane 3 u03b1 20 u03b1 diol 100 pregnanediol pregnanediol glucuronide 13320 u03b1 hydroxy 4 pregnen 3 one 126 20 u03b1 reagent 5 u03b2 pregnane 3 u03b1 20 u03b1 diol 05 u03b2 pregnane 3 u03b1 17 u03b1 20 u03b1 triol 05 u03b2 pregnane 3 u03b1 20 u03b1 diol diacetate 0progesterone 0estradiol 0estriol 0androsterone 0cortisol 0corticosterone 0______________________________________ the 20 u03b1 reagent can be bound to glass fiber paper after reacting thecleaned paper with 10 v v 3 aminopropyltriethoxysilane in toluene the free amino groups in this product can be cross linked in 2 5 glutaraldehyde to polylysine mol wt about 50 000 this provides a suitable support for covalently binding c 3 coupling derivatives of 20 u03b1 for this purpose the 3 0 carboxymethyl oxime can be preparedin high yield by treatment of the 20 u03b1 steroid with aminooxyacetate in pyridine.