SummaryLegionella pneumophilaantibodies (serogroup 1) were measured by means of indirect immunofluorescence in sera of two groups different-aged persons: on the one hand among 729 Belgian twenty years aged soldiers and on the other hand among 290 persons more than fifty years old. In neither of these groups theLegionella pneumophilaantibodies titre reached the positivity limit: 1/256. Nevertheless with lower rates, a different prevalence is observed among both groups of people.