The responses of cats to Brugia pahangi, which parasitizes them in nature, mimic those of humans to Brugia malayi and Wuchereria bancrofti in many important respects. It is likely that many of the features of the relationship between host and parasite that can be studied in cats infected with B. pahangi also apply to humans, but for a variety of practical reasons cannot be demonstrated in humans. Both immunologically and parasitologically there is a profound difference between B. pahangi-infected cats that have microfilariae in their blood and those that either do not become microfilaraemic or clear their blood of microfilariae and become post-microfilaraemic. Microfilaraemic cats are susceptible to reinfection and fail to recognize the surface of the sheath of microfilariae. They also do not produce antibodies against several components recognized by post-microfilaraemic cats. Cats that have destroyed their microfilariae also destroy their adult worms and are very resistant to challenge with infective larvae. Apart from the unique ability to recognize the sheath of microfilariae in fluorescent antibody tests, sera from these cats react with microfilarial antigens at 61-81 kDa, antigens of infective larvae at 22 and 18.5 kDa and adult antigens at 34, 18, 16, 13 and 11.5 kDa. None of these antigens are recognized by microfilaraemic cats.