Based on the partial sequences of the specific DNA cloned fragment from T. gondii (ZS2 strain) a specific primer of the oligonucleotide for the Toxoplasma gondii DNA sequence has been designed and synthesized in our laboratory. The method of the DNA diagnosis for toxoplasmosis by polymerase chain reaction (PCR) has been established. A specific amplified band was shown in the PCR products from DNAs of T. gondii and seven manifold terata. The DNAs from the peripheral blood leukocytes of fifty normal individuals and seventy-five patients as infants with hepatitis syndrome and pregnant women with previous abnormal birth histories were diagnosed by PCR. Among the seventy-five diagnosed cases, ten were positive. The normal individuals all were negative. Using 32P-cloned T. gondii specific DNA fragment as probe and Southern blot assay, the results showed that the probe only hybridized to the specific amplified DNA bands, but did not hybridize to the amplified DNA products of negative cases. Our PCR method is a rapid, highly specific and sensitive one for detecting toxoplasmosis as compared with DNA probing, immunoassay and animal inoculation.