Media conglomerate Cox Enterprises, after considering a lead role as venture capital investor in ad-network creator Adify, bought the company Tuesday for $300 million.

San Bruno, California-based Adify creates niche vertical ad networks for companies like Gay Ad Network, aimed at the lesbian and gay market, and Yardbarker, whose blogs reach sports fans.

The deal presents a rapid exit for Adify’s backers, which include Venrock Associates, U.S. Venture Partners, GE Commercial Finance, NBC Universal and Time Warner Investments. All told, 3-year-old Adify took in $27 million in two venture rounds.

Brian Ascher, general partner at Venrock, said Adify began discussing an advertising network deal with Cox about a year ago.

“That progressed to them leading the Series C, which never happened,” he said, and ultimately to the acquisition.

Atlanta-based Cox, which expects to complete the acquisition in May, is a $15 billion company with an array of media properties including cable TV systems; newspapers like the Atlanta Journal-Constitution; TV stations, including KICU in San Jose, California, and KIRO in Seattle Washington, and radio stations in markets like Houston, Miami and Orlando, Florida.

Adify, whose white-label technology lets clients create their own vertical ad networks, will be operated as a stand-alone company, retaining Russ Fradin, president and co-founder, at the helm.

Mr. Ascher said that when Venrock led Adify’s first venture round, “we thought it had good legs and it could be a standalone company.” Adding to the attraction, however, was the possibility that “Google, Yahoo, Microsoft or the dozens of media companies” could take a strategic interest in the startup.