The purpose of these experiments was to define the significance of the microfilarial stage to the hyporesponsive condition seen in lymphatic filariasis. Two types of experiments were conducted with Brugia pahangi-infected gerbils. In one, in vitro lymphocyte blastogenesis and in vivo granuloma formation in response to parasite antigen were correlated to microfilaremia in chronically infected individuals. In a second set of experiments, the level of in vivo granuloma formation was assessed following chemotherapeutic removal of microfilariae with ivermectin. The results indicated that the microfilarial stage alone is not responsible for the maintenance of the low cellular responses seen during chronic infections in this model. Furthermore, the data suggest that the degree of downregulation of these responses may be related to parasite burden.