The courses of IgG and IgM antibody levels against adult worm and microfilarial antigen were determined in isogenetic cotton rats infected quantitatively with Litomosoides carinii. Against both antigens, IgG as well as IgM, antibody levels exceeded significantly those of noninfected animals, and the IgG levels were generally higher than the IgM levels. The total antibody production was depressed transiently 8 weeks p.i. by the appearance of microfilariae in the peripheral blood. A second transient depression occurred two weeks earlier against microfilarial antigen than it occurred against adult worm antigen. At 3 different times after the infection, the amount of adsorbed antibodies was assessed on adult worms and on microfilariae, which were isolated from blood and by a specially developed method from the internal organs. The percentual distribution of microfilariae in the different organs and blood changed during patency. In spleen, kidney, and lung a continuous increase was observed, whereas in heart and liver the percentage initially increased, and then decreased. The opposite course was seen in blood. Most antibodies adsorbed on the surface of microfilariae and adult worms belonged to the IgM class. Predominantly, the antibodies were detected on organ microfilariae, particularly on those in spleen and kidney, while blood microfilariae had lower amounts of bound antibodies. Furthermore, by complement fixation, no common antigens could be detected on the surface of host cells and the different L. carinii stages.