Sir Paul McCartney's wife Heather has visited the New York office of Jennifer Lopez's fashion company Sweetface to protest against its use of fur.
Mills McCartney is a patron of the Adopt-A-Minefield charity
"I just want to raise awareness about the skinning alive of all the animals in China," she told reporters.
"The next stage is I'll find out where she lives and show up there."
Earlier this week Mills McCartney was asked to leave a J Crew clothes store after walking in wearing a flat-screen TV showing images of animal cruelty.
Wednesday's protest, timed to coincide with New York Fashion Week, saw the 37-year-old campaigner deliver a DVD to the Sweetface offices and hold an impromptu press conference.
According to photographers at the scene, it was followed by an altercation with security staff.
"I try and do everything gently and diplomatically, and then what do you do?" she said.
"Let millions of animals continue to be skinned alive?"
Mills McCartney is a United Nations Association Goodwill Ambassador and a patron of the Adopt-A-Minefield campaign.
Earlier this year she made an appearance on US daytime TV soap Days of our Lives to promote the anti-landmine charity.
She and her husband have a daughter, Beatrice, who was born in October 2003.