Viral replicon selected nucleic acid expression libraries are useful for analyzing multiple antigens associated with a parasite, pathogen or neoplasia or for preparing immunogenic compositions for generating immune responses specific for the parasite, pathogen or neoplasia. Alphavirus replicon particles representative of the nucleic acid expression library are preferred. The nucleic acid library can be a random library, or it can be prepared after a selection step, for example, by differential hybridization prior to cloning into the replicon vector.