A series of 925 clinical isolates of klebsiella was examined by serological and biochemical typing. To perform serological typing (capsular swelling) 77 capsular antisera were prepared, tested against the type strains and grouped in 13 pools. With this serotyping method 80% of the cultures were typable and 63 distinct types could be recognized. All strains were typable biochemically by means of the numerical coding system of the API-20E system supplemented by digits derived from 15 additional conventional biochemical tests. With the API-20E system 24 different biotypes could be distinguished whereas the combination of API-20E and the 15 additional tests produced 93 biotypes. Maximum discrimination of strains was achieved by the combination of serological and biochemical typing (256 bioserotypes). The reproducibility, typability and discriminating power of the biotyping system was not inferior to serotyping. For epidemiological purposes biotyping can replace serotyping of Klebsiella species, especially in laboratories less well equipped.