The BSS (Okapi search engine) is the platform for all current Okapi projects, having been under development since 1987. It supports a low-level query language based on ISO SR (Z39/50) but incorporating probabilistic term weighting and document ranking. Other BSS functions relevant for the ENQUIRE project are term stemming, stop-word identification, and special searches on ``controlled-language'' (thesaurus) terms in the descriptor fields of documents.