Saline antigen extracts of microfilariae, adult worms and third-stage larvae of subperiodic Brugia malayi maintained in gerbils were prepared for use as skin test reagents. Patients were studied on three different islands in the Philippines, one endemic for Bancroftian filariasis (Sorsogon, Luzon), another endemic for Malayan filariasis (Palawan) and the third without endemic filariasis (Cebu). A dose-response curve was established initially in patients with Bancroftian filariasis: thereafter 1.0 microng of the B. malayi antigens and 0.05 microng of Dirofilaria immitis FST antigen (obtained from Dr. T. Sawada) were used. Sizes of reactions were measured by recording the diameters of wheals at 20 minutes, 24 and 48 hours. There was a very high correlation in immediate hypersensitivity reactions among the three B. malayi antigens. Reaction sizes followed a normal distribution. When an area of an antigen-induced wheal 3 X that of the saline control was considered a positive reaction, 99% of 150 patients with Bancroftian filariasis and 96% of 45 subjects with Malayan filariasis reacted to B. malayi larval antigen. Only 68% of patients with Bancroftian filariasis but 90% of those with Malayan filariasis reacted to D. immitis FST antigen. There was no relationship between skin reactivity and age, sex, microfilaremia or severity of clinical disease. Approximately half of 50 patients who lived in an endemic area for W. bancrofti but had neither patent infection nor clinical disease reacted to B. malayi antigens. A maximum of 7% of 120 age- and sex-matched controls from Cebu gave false positive reactions with any of the antigens. Only a small proportion of patients gave 24- and 48-hour reactions. It is concluded that the use of antigens prepared from a human parasite, subperiodic B. malayi, which is easily maintained in a laboratory animal host, improves the ability to diagnose filarial infections by immunological means.