An internal control was used in a polymerase chain reaction (PCR)-ELISA-based technique to detect the Hha I repeat of the filarial parasite Brugia malayi. A single microfilaria added to 200 microl of blood was reliably detected. The assay was evaluated on field samples from persons living in an area endemic for Anopheles-transmitted, nocturnally periodic B. malayi in central Sulawesi, Indonesia. Examination of night blood of 138 individuals for the presence of microfilariae by filtration revealed 44 microfilaria carriers. All microfilaria carriers were also positive in the PCR-ELISA and, in addition, 14 more samples were proven to contain parasite DNA. The sensitivity of both methods was compared on night and on day blood samples collected from 113 persons. Whereas 37 microfilaria carriers were identified by filtration of night blood, no microfilariae were observed in the corresponding day blood samples. The PCR-ELISA result was positive in all 37 night blood samples of microfilaria carriers and in an additional 13 night blood samples without microfilariae. Parasite DNA was detected in 31 day blood samples of microfilaria carriers and in 3 day blood samples of amicrofilaremic persons. Assuming a sensitivity of the PCR-ELISA on night blood of 100%, the sensitivity of night blood filtration is 74% and that of the PCR-ELISA on day blood is 68%. These data suggest that the described PCR-ELISA method is capable of detecting infections with nocturnally periodic B. malayi in day blood samples. Therefore, this method may facilitate both the identification of endemic areas and the monitoring of control programs.