The sensitivity and specificity of the polymerase chain reaction (PCR) method was studied in vitro with HeLa cells infected with Chlamydia trachomatis serovar L2. Three different primer sets were studied; they were derived from the endogenous plasmid, the nonvariable part of the MOMP gene and the 16S ribosomal RNA (rRNA) gene. The plasmid primers were the most sensitive in the PCR method and detected at least 0.1 infectious unit of C. trachomatis in the presence of a superfluous amount of human DNA. Application of this plasmid PCR to 13 C. trachomatis culture-positive cervical smears containing < 10- > 200 inclusion-forming units showed that it was the most sensitive of the three methods and detected C. trachomatis in all samples. This correlates with the observation that the plasmid PCR method could detect C. trachomatis in cervical smears of four symptomatic patients for up to 3 weeks after the start of treatment with doxycycline. In contrast, the MOMP gene- and rRNA gene-directed PCR, as well as culture and direct immunofluorescence, gave negative results within 1 week. Therefore, we conclude that the plasmid primers are the best candidates for use in the PCR method in C. trachomatis screening programmes and clinical follow-up studies.