Multiplayer focused first-person shooters Payday: The Heist and Left 4 Dead are blending together, Overkill has announced.
The Swedish developer, now owned by Syndicate maker Starbreeze, described the mysterious collaboration as "in-depth".
"As perceptive gamers will have noticed, several hints have recently been dropped into Payday The Heist, which has led to various rumours," Overkill game director Ulf Andersson said.
"We are excited to be able to confirm that an in-depth collaboration between Overkill and Valve is currently in production.
"We are working on a very cool blend of Payday and Left 4 Dead. I am sure it is so exciting that it will have some players check into the hospital before we are done."
Last month Starbreeze told Eurogamer Overkill was hard at work on "enhancing" Payday, as well as making a sequel. Overkill promised more news soon.