Capillaria hepatica was found in 57% of rats captured in the city of Salvador, Bahia - Brazil. Worm eggs were then isolated from animais with natural infection and used to infect albino mice experimentally. Circulating antibodies were demonstrated by immunofluorescence in the sera of mice during the course of experimental infection, from the first to the 5th week following inoculation. Such antibodies were seen to bind to eggs, larvae and worms present in the liver. These preliminary data suggest that immunological tests may be of value to detect human cases with non-fatal infection caused by Capillaria hepatica.