In online ad auctions for ad space, it’s sometimes useful to prevent showing an ad to certain audiences, a concept known as negative targeting. For example, you might not want to show a new customer advertisement to existing customers. New customer acquisition campaigns most often have this as a critical requirement. Protected Audience currently enables ads to target users that have been joined to a given interest group through some past activity on the web. This feature extends Protected Audience to enable negative targeting by allowing new ads to target only those users who have not been joined to a given interest group. In this way, we're enabling advertisers to target new groups of users using the existing privacy-preserving concepts of the Protected Audience API. The HTTP response header processing involved in this feature works with Fetch and iframe navigations.