The polymorphic GC-rich repetitive sequence (PGRS) found on the chromosome of Mycobacterium tuberculosis was characterized by means of mapping, cloning and sequencing. PGRS was present in at least 26 loci and consisted of many tandem repeats of the consensus sequence CGGCGGCAA. As the core of the consensus motif was the triplet CGC, or CRR (where R is a purine), it seems likely that PGRS arose by means of triplet expansion, accounting for its polymorphism. Several copies of PGRS were linked to a conserved open reading frame. PGRS was used as the target sequence for the polymerase chain reaction in an attempt to develop a new typing technique.