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Legionella pneumophila serogroup 1 can be subgrouped by panels of monoclonal antibodies. The Oxford panel divides serogroup 1 strains into three major subgroups, provisionally named Pontiac, Olda and Bellingham. Strains causing outbreaks tend to be in the Pontiac subgroup, with strains from the other subgroups appearing also in sporadic cases. Isolates from sources in the UK unassociated with outbreaks are mainly the Olda and Bellingham strains; only 10% are Pontiac, yet this strain has caused 80 to 90% of the cases of Legionnaires' Disease so far studied. We compared our method with other methods of subgrouping serogroup 1 strains.