Food safety watchdog in Shanghai has launched a probe over the "rubber eggs" that has hit the markets across China. The eggs contain hard yolks which bounce like rubber after being boiled.
The investigation was launched after some consumers said they bought the "artificial" eggs made by traders seeking profits, the China Daily said.
But food experts said the eggs were likely to be natural. Their unusual characteristics could be explained by high levels of a compound called gossypol.
Gossypol normally exists in the residue of cotton seeds added to chicken feed as an extra protein source.
Experts say large doses of the compound suppress sperm activity. Gossypol has been used in male contraceptive pills in a couple of countries.