Adult Hooded Lister rats were given 5000 Nippostrongylus brasiliensis larvae on day 3 or 7 after infection with Trypanosoma brucei and a second dose of 5000 nematode larvae 28 days later. A similar number of rats was infected only with N. brasiliensis larvae. Comparison of antibody levels in serum and the respiratory and alimentary tracts showed that T. brucei infection influenced both systemic and local antibody responses of rats to N. brasiliensis antigens. After primary infection systemic antibody responses were mainly impaired, the level of suppression depending upon the interval between trypanosome and nematode infections. Anamnestic responses were diminished in both antibody systems. The number of worms reaching the small intestine of T. brucei parasitised rats after primary infection was twice- and after reinfection three-times higher than in rats subjected to nematode infections alone. However, adult nematode expulsion was not delayed. The results suggest that N. brasiliensis infection causes a multiantigenic stimulation of both systemic and local humoral responses of the host. Furthermore, they indicate that depression of systemic antibody responses may enhance worm establishment.