Sequential blood samples collected from mice infected perorally with an avirulent strain of T. gondii were analysed for parasite DNA by a polymerase chain reaction method (PCR). Two pairs of primers specific for gene B1 and the repetitive DNA sequence TGR1E were used for DNA amplification. Amplified products were detected by means of electrophoresis with ethidium bromide staining. Parasitemia was also determined by cell culture. Parasitemia was never detected by the tissue culture method, whereas parasite DNA was continuously detected with PCR from day 2 to day 21. These results confirm the high sensitivity of PCR for T. gondii DNA in blood, and show that circulating DNA is present for long periods in mice following primary infection.