Sandra Bullock, Kevin Hart and other celebrities are lending their weight to the Tropical Storm Harvey relief effort in Texas.
Bullock has donated $1m (£774,000) to the American Red Cross in the wake of the flooding in Houston and beyond.
Hart has called on his famous friends to dig deep, while his own fundraising campaign is near $1m.
Others to have pledged money include The Kardashians, Jennifer Lopez, Chris Brown and Dwayne “The Rock” Johnson.
Sandra Bullock – $1m
American Red Cross communications vice president Elizabeth Penniman said: “It’s times like this when we do receive such an incredible amount of support. Especially during times of disaster, people see what’s happening in Texas and our hearts all go out to them.
“Having someone like Sandra Bullock make this kind of commitment, it helps bring people together and open their hearts and be even more generous.”
Kevin Hart – $50,000
The Ride Along and Get Hard star posted a video pledging $25,000 (£19,400) – which he later doubled – and named a string of other A-listers whom he said should follow his example.
He said: “I think we’ve participated in a lot of challenges on the internet, some meaningful, some meaningless, but we’ve all done it… At this point I’m going to start a real challenge. I’m challenging a lot of my celebrity friends to follow my lead.”
The Kardashians – $500,000
Kim Kardashian West responded: “@kevinhart My mom, sisters & I accept your challenge. We will be donating $500,000 to @redcross and @salvationarmyus today #HoustonStrong”
Dwayne ‘The Rock’ Johnson – $25,000
The movie star said: “I want to send a message to the people of Houston – you’ve got to stay strong and you’ve got to hold on to faith.”
Jennifer Lopez and boyfriend Alex Rodriguez – $25,000 each
Also on Instagram, the actress told fans: “We’ve been watching everything that’s been going on down in Houston and our hearts are just breaking. It’s just devastating.”
Chris Brown – $100,000
“Kevin Hart I accept your challenge,” the singer announced. “We need to come together.”
But he added that he was “sceptical” about the Red Cross, saying “my donation will go to the people” instead. He didn’t explain how that would work though.
TI – $25,000
The rapper added his donation – but was even more vehement in insisting it wouldn’t go to the Red Cross.
Jensen Ackles – $50,000
The Texas-based Supernatural actor (above, centre) sent his “thoughts and prayers to everyone getting hit by hurricane Harvey… Including my wife and children”.
With his co-stars, he then started a crowdfunding campaign that has so far raised more than $200,000 (£155,000).
He said: “I wish I was down there and helping people and pulling people off of roofs in a boat right now but I’m not. This is what I can do.”
Chris Young – $100,000
The country singer kicked off his own campaign, which now stands at more than $300,000 (£232,000).
He said: “I have friends and family there and I’m almost certain my house down there may have to be torn down as it was in one of the areas that was hardest hit by winds and flooding… But that’s the least of my concerns.”
Nicki Minaj – $25,000
She posted: “I’ll donate 25K for Houston. Praying for everyone there. Great work @kevinhart4real”
DJ Khaled – $25,000
The hit-maker added his name to the cause: “Kevin Hart I hear you loud and clear. I just sent $25,000 to Houston, Texas to help all the families in need… All my celebrity friends, let’s go.”
The Chainsmokers – $30,000
The pop duo said: “We didn’t officially get called out by Kevin Hart but we felt really compelled to donate to his Hurricane Harvey fund.”