Pearl Jam guitarist Mike McCready also joined the duo for the 'beautiful session.'

Man, this Paul McCartney guy really gets around.

After his work with Kanye West and Rihanna -- including the just-released "FourFiveSeconds" video -- the Beatles legend is teaming up with another relative rookie: Lady Gaga.

Rihanna, Kanye West & Paul McCartney Release 'FourFiveSeconds' Video

"Had a beautiful session with Sir Paul McCartney and friends," Gaga wrote on Instagram, alongside a photo of herself, Macca and a room full of instrumentalists -- including none other than Pearl Jam guitarist Mike McCready, on Gaga's right. "Working on one of his many secret projects! Killer musicians, vibe, and lots of laughs."

Had a beautiful session with Sir Paul McCartney and friends. Working on one of his many secret projects! Killer musicians, vibe, and lots of laughs. A photo posted by @ladygaga on Feb 3, 2015 at 5:57pm PST

But that wasn't the only photographic tease: Gaga also posted a solo pic of just her and McCartney, recalling the first time she heard from the rock-and-roll icon -- and promptly hung up on him.

"Always a good time with my buddy," she wrote. "I'll never forget when he called me last year to work and I hung up the phone cuz I thought it was a prank!"

Always a good time with my buddy. I'll never forget when he called me last year to work and I hung up the phone cuz I thought it was a prank! A photo posted by @ladygaga on Feb 3, 2015 at 6:00pm PST

So how will this collaboration turn out? Will it be only his guitar work, in the vein of his recent work with Kanye and Rihanna? Or maybe Gaga has her sights set on a new partner, after her No. 1 duets album with Tony Bennett? We'll have to wait and see.

Requests for further details from McCartney's rep went unreturned at press time.