Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for distribution of co-sponsored content items. In one aspect, a method includes receiving a first bid from a first content distributor and a second bid from a second content distributor, where each of the bids specifying respective amounts that the different content distributors will pay for distribution of respective content. Using the first and the second bid, a total bid is computed for a co-sponsored content item that includes the first content and the second content. A request is received for content to be presented in a presentation slot of a publisher property and the co-sponsored content item is selected for presentation based on the outcome of an auction performed using the total bid as one of a plurality of bids. In turn, data that cause presentation of the co-sponsored content item in the presentation slot of the publisher property are provided.