Progesterone reinitiates in vitro meiosis in Xenopus laevis oocytes. This action of the hormone can be abolished by the exotoxin of vibrio choletae. The concentration of toxin which inhibits 50% of the progesterone (10 μM) action is about 2.5 pM. .

Binding experiments using ¹²⁵I labelled toxin demonstrated the existence of high affinity binding sites (K_D ~ 0.2 nM) located probably on the surface of the oocytes.