From 22 patients with Legionnaires' disease, 86 sera were examined for specific serotype 1 IgM and IgG antibodies by the indirect immunofluorescence technique.

No antibody was detectable until 8 days or more from the onset of symptoms. When produced the amount was widely variable and remained detectable for periods from less than 34 days to more than 1 year.

Initially IgM antibody predominated, ten patients produced only IgM in the first 21 days, six produced only IgM in the first 28 days and three did not produce IgG at any time. One patient, and possibly a second, produced only IgG antibody.

Since IgM antibody was still present in one patient after a year it is important not to accept the presence of this as evidence of very recent infection.

It is advisable that any type of serological test for L. pneumophila infection should detect the production of both IgM and IgG antibodies.