A novel real-time quantitative polymerase chain reaction (RTQ-PCR), based on the 18S ribosomal RNA genes of P. falciparum NF54, was used to evaluate the level of parasitaemia in 5 human volunteers experimentally infected by mosquitoes [Netherlands; date not given]. All 5 volunteers developed parasitaemia, as determined by thick smear after a mean of 9.1 days (range: 7.6-10.3), and fever accompanied by mild to moderate symptoms. After anti-malaria treatment with chloroquine, the clinical course of the infection was uncomplicated, with resolution of the symptoms within 2-3 days and clearance of parasitaemia within 2 days as detected by thick smear. With the RTQ-PCR, circulating P. falciparum parasites were detected in 4 of the 5 volunteers as early as 6.3 days after infection (parasite densities: 20-147 per ml blood). During the incubation period, the number of malaria parasites steadily increased, with a temporary but significant decrease at 20-24 h after detection of the first parasites, to a mean maximum of about 9000 parasites per ml of blood (range: 1485-18 264) and dropped after the start of chloroquine treatment. The number of parasites detected by thick smears was corrected for the number of leukocytes and showed a good correlation with the numbers estimated by the RTQ-PCR (R=0.82). These results demonstrate that this RTQ-PCR is a specific and sensitive quantitative assay for malaria parasite detection.