Kayak has quietly introduced mobile display ads into the search results screens of its iPhone app.
Launched in mid-May, the initial advertisers include Holiday Inn Express, InterContinental Hotels, hotels.com and CarRentals.com.
The display ads appear at the bottom of search results pages in the Kayak Flights iPhone app.
So, while searching for flights, travelers might consider a hotel or car rental from these advertisers.
Kayak's sales effort for mobile advertising is an in-house effort, officials say.
The move comes as Kayak attempts to ratchet up its mobile traffic with the pending introduction of a booking assistant, Kayak Connect.
Kayak already sources about 10% of its global traffic from its mobile apps.
The hope is that mobile advertising will become a revenue stream, as well.