Private-equity titans Henry Kravis and Steven Schwarzman are teaming up on what is likely the biggest leveraged buyout in several years.

KKR has joined an investor group of Blackstone, Carlyle, TPG Capital and Temasek to bid more than $12 billion for Life Technologies, a source said.

Suitors must make “binding” offers by today for the maker of laboratory equipment. KKR declined comment; Blackstone did not return calls.