To study immunological responses in filarial infections, interleukin-2 receptor α (IL-2Rα) expression in the peripheral blood lymphocytes (PBL) of Lewis rats infected with Brugia pahangi was observed by flow cytometry. During infection, no increase in IL-2Rα was observed in either microfilaraemic or amicrofilaraemic rats. The PBL of rats were stimulated with phytohaemagglutinin (PHA) and the frequency of IL-2Rα-positivity was examined. After microfilariae appeared, the increase in the rate of IL-2Rα-positivity in microfilaraemic rats was less than in amicrofilaraemic rats. The anergy of IL-2Rα expression in microfilaraemic rats was restored when their lymphocytes were cultured with normal rat serum. IL-2Rα expression in normal uninfected rats decreased when cultured with serum from microfilaraemic rats. These results suggest that microfilariae-related factors in rat serum suppress PHA-induced IL-2Rα expression in PBL and thus inhibit cell proliferation and host immunity.