Sera from 637 Athens residents of various age groups were examined by plaque reduction neutralization test for antibodies against Naples and Sicilian Phlebotomus fever viruses. A marked change in the prevalence of antibodies to both agents was observed in persons born after 1.946, when residual insecticide spraying for malaria control was initiated in Greece. The prevalence of Naples and Sicilian neutralising antibodies among residents >=3O years of age was 36% and 13%, respectively. In contrast, only 4% of persons <=29 years of age had Naples antibodies and all were negative to Sicilian. These serologic data confirm previous clinical observations that sandfly fever became uncommon in Athens after initiation of the insecticide spraying program. Presumelly the spraying program was effective in reducing the Plebotomus population to levels where virus transmission was minimal. New Information on the specificity and duration of Phlehotomus fever neutralizing antibodies is also presented.