Immunogenic midgut antigens of the human body louse, Pediculus humanus humanus L., were localized using rabbit antisera against a louse-midgut extract followed by a 2nd antibody conjugated to either fluorescein or colloidal gold. Strong fluorescence was observed on the outer membrane of the epithelial cell of the midgut. The immunogold technique revealed that most of the antigens were localized on the microvilli of the midgut cells. Small numbers of gold particles were also seen in the gut lumen and within the cell cytoplasm. Only a few gold particles were seen in the lumen of the gut sections incubated with control sera.