Ms Moskalenko represents several Kremlin critics
Fears that a lawyer representing several critics of Russia's government may have been poisoned appear unfounded, French authorities say.
Karinna Moskalenko fell ill in the French city of Strasbourg, and traces of mercury were found in her car.
She and members of her family were treated for nausea and headaches.
But a French prosecutor says it seems the mercury came from a broken thermometer or barometer, and was spilled by the car's previous owner.
The vehicle was bought last August by Ms Moskalenko's husband, from an antique dealer who used it to carry his wares.
On investigation, it turned out that an old thermometer or barometer had been broken accidentally during a journey.
Ms Moskalenko's clients include the jailed former Russian oil tycoon, Mikhail Khodorkovsky, and the family of murdered journalist Anna Politkovskaya.
Fears had been aroused that she had been subject to an attack similar to the one that killed former Russian agent Alexander Litvinenko in London in 2006.