A method and system for introducing spam into a search engine for testing purposes is provided. An active spam testing system receives from a tester a specification of spam that is to be introduced into the search engine for testing purposes. The testing system may then generate auxiliary data structures for storing indications of the spam that is to be introduced. A search engine has original data structures that may include a content index and a link data structure. The testing system stores the indications of the spam in the auxiliary data structures so that use of the search engine for non-testing purposes is not affected. When the search engine is used for testing purposes, the search engine generates search results based on a combination of the original data structures and the auxiliary data structures.