The titer of anti-Culex quinquefasciatus antibodies which were mostly of IgG and IgE isotypes was determined in humans living in Wuchereria bancrofti endemic regions. A progressive increase in the titer of antibodies was observed with age. In contrast, sera from people living in regions where the Cx. quinquefasciatus is of low prevalence were found to have markedly reduced IgG and almost zero IgE levels.