SRS (Sequence Retrieval System) is an information indexing and retrieval system designed for libraries with a flat file format such as the EMBL nucleotide sequence databank, the SwissProt protein sequence databank or the Prosite library of protein subsequence consensus patterns. SRS supports the data structure of these libraries by providing special indices for implementing lists of subentities (e.g. feature tables) or hierarchically structured data-fields (e.g. taxonomic classification). A language (ODD) has been designed for the convenient specification of library format and organization, representation of individual data-fields within the system (design of indices) and structuring other data needed during retrieval. This ensures flexibility required for coping with different library formats, which are subject to continuous change. Queries and inspection of retrieved entries can be performed from a user interface with pull-down menus and windows. SRS supports various input and output formats but is particularly well adapted to the GCG programs.