The intradermal test using as antigen a 1 per cent saline extract of Dirofilaria immitis powder was performed in Singapore on 69 persons with eosinophilic lung, 32 with mild eosinophilia, 49 with filariasis, 75 normal Asians, and 66 normal Britishers. The test was positive in 100 per cent of the cases of eosinophilic lung, 73.5 per cent of the filariasis group, 59.4 per cent of cases of mild eosinophilia, 53.3 per cent of normal Asians, and 4.5 per cent of the Britishers. The filarial complement fixation test using a 1 per cent alcoholic extract of the same antigen gave a positive rate of 100 per cent in the eosinophilic lung group, whereas only 24.5 per cent of the filariasis patients gave a positive reaction. Skin sensitivity to D. immitis antigen persisted in the cases of eosinophilic lung even when the previously positive serologic reactions had become negative following treatment with diethylcarbamazine. Therefore, the intradermal test cannot be useful in the diagnosis of either filariasis or eosinophilic lung in Singapore. In view of the skin sensitivity to a filarial antigen demonstrated in patients suffering from eosinophilic lung, the etiologic possibility of an infection by a species of filarial worm found normally in nonhuman hosts is discussed.