Experiments were conducted to measure the degree of lymphatic pathology which develops in mongolian jirds (Meriones unguiculatus) sensitized to Brugia pahangi antigens prior to subcutaneous infections. Two protocols were used to sensitize jirds. One group of animals received three intravenous (IV) inoculations of 5,000 frozen, washed, B. pahangi-microfilariae at 10-day intervals. A second group received three inoculations of 150 micrograms of soluble somatic adult B. pahangi antigen (Ag) in Freund's complete adjuvant (FCA) at 10-day intervals. Groups of animals receiving saline in FCA and animals receiving no treatment served as controls. Following immunizations, animals from each group were tested for circulating antibody by the indirect hemagglutination assay, and for immediate and delayed hypersensitivity responses by the footpad swelling assay. All sensitized animals tested showed positive reaction to these assays. Observations at necropsy 90 days after inoculation with infective larvae showed that: 1) the percent recoveries of adult worms were the same in all treatment groups; 2) the numbers of patent infections which developed in the Ag in FCA-treated animals was greatly reduced; 3) level of microfilaremia which developed in animals sensitized with microfilariae was markedly lower; and 4) the degree of lymphatic pathology as judged by lesion score, numbers of intralymphatic thrombi, and lymphatic vessel size was significantly greater in presensitized animals than in nonsensitized infected controls. The increased lymphatic pathology seen in presensitized animals was most marked in jirds with occult infections and high antibody titers. These observations indicated that the B. pahangi-jird model is a useful semiquantitative system for the study of filarial-associated lymphatic pathology and strongly supports the hypothesis that the host immune response is involved in the pathogenesis of lymphatic filariasis.