We studied the sensitivity of polymerase chain reaction (PCR) for detecting DNA of migratory larvae of Trichinella spiralis at an early stage of infection with this parasite. We derived primers for PCR from a 1.6-kb repetitive sequence of the genome of T. spiralis and used PCR to detect Trichinella-specific DNA in blood of mice infected with 20, 100, or 300 muscle-derived larvae of T. spiralis at 3-21 days postinfection (dpi). We detected T. spiralis DNA in blood of mice infected with 20 larvae at 5 and 6 dpi, with a detection rate of 7.69% and in blood of mice infected with 100 larvae at 5-12 dpi, with a peak detection rate of 38.46% at 7 dpi. PCR detected T. spiralis larvae at 5-17 dpi in mice infected with 300 larvae, with detection rates exceeding 50% from 5 to 10 dpi and a peak rate of 61.54% at 7 dpi. The detection rates of T. spiralis larvae with PCR in the three groups of mice showed an increasing trend with an increase in the infecting dose of larval parasites (F = 17.811, p < 0.01). Our findings indicate that the sensitivity of PCR for detecting DNA migratory larvae of T. spiralis in blood of mice infected with this parasite depends on the severity of infection and the time elapsed after infection, and suggest that PCR may be useful for detecting Trichinella infection at an early stage in humans and food animals that test negatively for anti-Trichinella antibodies.