The antigenic properties of adult male and female worms of the nematode Dipetalonema viteae (Filarioidea) were studied by immunocytochemistry. Using sera of rodents infected with this parasite, the binding of antibodies to sections of worms embedded in Epon was assayed by the peroxidase-antiperoxidase (PAP) technique and by light microscopy. In female worms antigen-antibody reactions were located in the uterine fluid, the inner uterus wall, the circumference of the microfilariae and in the egg shell. Male D. viteae showed binding of antibodies in the vesica seminalis and in the vas deferens. Both sexes has also less pronounced antigenic activities in cuticula, muscle and intestine.