Shares in government-controlled Aeroports de Paris, operator of Charles de Gaulle and Orly airports, soared after French construction company Vinci said it had acquired a 3.3 percent stake.
ADP shares were up 8.8 percent at 76.17 euros ($112.13) in Paris midday trading. Vinci shares fell 1.6 percent to 49.83 euros ($73.35).
Vinci said in a statement it does not intend to increase its stake "unless there was a change in the government's shareholding in the future."
The government owns 68 percent after selling shares 18 months ago to raise money for airport expansion. The Finance Ministry, which runs the government's share portfolio, was not immediately available for comment.
"Vinci is signaling to the market their clear interest," said Exane BNP Paribas analyst Steven Fernandez. "The company is positioning itself for an eventual privatization of ADP in the future."
Vinci said it made the purchase to compliment an airports portfolio that includes airports in the Cambodian capital, Phnom Penh, and three French airports.
It is not the first time Vinci has expressed interest in government assets. In 2005, Vinci was the successful sole bidder for a stake in highway concession manager Autoroutes du Sud de la France, sold by the government.
If ADP is privatized, Vinci will likely face steeper competition from construction companies such as Australia's Macquarie Infrastructure Group, Fernandez said.
A further share sale of ADP could be popular. The weekly financial newspaper Le Revenu speculated on possible share sales for 10 state-owned companies after Nicolas Sarkozy's election as French president, but recommended buying only one: ADP.