The straim M 20 was isolated y Borg-Peterson (1938) from a patient in Demmark. It was shown by agglutin-absorption tests to differ serologically from strain RGA. The strain M 20 is able to absorp nearly all antibodies from RGA antiserum whereas strain RGA only removes its homologus antibodies from M 20 antiserum. The strain was therefore considered to represent a 'subtype' of serovar icterohaermorrhagiae.
Gispen and Schuffner (1939) confirmed the serological results but they classified M 20 as the 'complete biotype' of icterohaemorrhagiae, and named it icterohaemorrhagiae AB. The strain was later considered to represent 'a subserotype' of icterohaemorrhagiae (WHO, 1965). Kmety (1966), who studied the antigenic structure of serovars of the Icterohaemorrhagiae serogroup, suggested abandoning the taxonomic rang of 'subserotypes' and proposed that M 20 should represent a 'serotype' named copenhageni. This suggestion was accepted by the TSC 1966 at their Moscow meeting in 1966(TSC meeting, 1966: Statements and Recommendations, 1971).