Sequences derived from the endogenous plasmid of Chlamydia trachomatis and from the genes coding for ribosomal 16S RNA of Chlamydia psittaci were used as primers and oligonucleotide probes for detection of chlamydiae by the polymerase chain reaction. The endogenous plasmid primers generated specific amplified products of 517 bp with all known Chlamydia trachomatis serovars. No specific products of Chlamydia psittaci and Chlamydia pneumoniae could be detected using these primers. With the rRNA primers specific amplified products of 208 bp were generated with Chlamydia psittaci, Chlamydia trachomatis and Chlamydia pneumoniae. No specific amplified products were detected with DNA isolated from a variety of microorganisms from the urogenital and the respiratory tract. Of 156 clinical specimens used for evaluation of the polymerase chain reaction, 26 were found to be positive for Chlamydia trachomatis on culture. All 26 culture positive samples were also found to be positive for Chlamydia trachomatis DNA by the polymerase chain reaction with both primer sets. Two culture negative samples were also found to be positive by this technique. The polymerase chain reaction thus seems to be a sensitive and reliable method for detection of Chlamydia trachomatis.