Inhibition of DB3 and Ab3 antiprogesterone–bovine serum albumin (BSA) sera with four progesterone conjugates. (a) Structures of conjugates: (from the top) progesterone-11α–hemisuccinyl (HMS), progesterone-6β–HMS, progesterone-3-carboxymethyloxime (CMO), progesterone-21–HMS. (b) Inhibition of DB3 (c) inhibition of serum 3 IgG and (d) inhibition of serum 4 IgG by progesterone-11α–HMS (•), progesterone-6β–HMS (▴), progesterone-3–CMO (□) and progesterone-21–HMS (▿).