We have successfully developed a restriction endonuclease fingerprinting (REF) technique for the study of meningococcal chromosomal DNA. A review of our application of this method in studies of meningococcal epidemiology and pathogenicity is given. By REF we could show that fingerprints of apparently identical B15 carrier strains were remarkably heterogeneous, whereas invasive B15 isolates possessed very similar fingerprints. We could also demonstrate REF heterogeneity among B15 isolates collected from cases with meningococcal disease and their close contacts during an outbreak of meningococcal disease in a military camp in North Norway. The REF technique has also proved valuable for our studies on meningococcal piliation and adherence. At present, we are studying 67 different meningococcal isolates collected from all parts of Norway during the MenOPP project.