A rapid method of assay, using a monoclonal antibody linked to alkaline phosphatase, was used for the detection of the Pontiac subgroup of Legionella pneumophila serogroup 1. It was tested for its specificity against 53 strains of Legionella recently isolated from the environment in Singapore and Malaysia. The specificity and sensitivity of this method of assay was confirmed, though there is some concern that the specificity was too narrow, and there are reservations about the criteria suggested for interpreting the results.