An inherent problem in the diagnostic PCR assay is the presence of ill-defined inhibitors of amplification which may cause false-negative results. Addition of an amplifiable fragment of foreign DNA in the PCR to serve as a hybrid internal control (HIC) would allow for a simple way to identify specimens containing inhibitors. Two oligonucleotide hybrid primers were synthesized to contain nucleic acid sequences of the Chlamydia pneumoniae 16S rRNA primers in a position flanking two primers that target the sequences of a 650-bp lambda phage DNA segment. By using the hybrid primers, hybrid DNA comprising a large sequence of lambda phage DNA flanked by short pieces of chlamydia DNA was subsequently generated by PCR, cloned into a plasmid vector, and purified. Plasmids containing the hybrid DNA were diluted and used as a HIC by adding them to each C. pneumoniae PCR test. Consequently, C. pneumoniae primers were able to amplify both chlamydia DNA and the HIC DNA. The production of a 689-bp HIC DNA band on an acrylamide gel indicated that the specimen contained no inhibitors and that internal conditions were compatible with PCR. Subsequently, a biotinylated RNA probe for the HIC was transcribed from a nested sequence of the HIC and was used for its hybridization. Detection of the HIC DNA-RNA hybrid was achieved by enzyme immunoassay (EIA). This PCR-EIA system with a HIC was initially tested with 12 previously PCR-positive and 14 previously PCR-negative specimens. Of the 12 PCR-positive specimens, 11 were reconfirmed as positive; 1 had a negative HIC value, indicating inhibition. Of the 14 previously PCR-negative specimens, 13 were confirmed as true negative; 1 had a negative HIC value, indicating inhibition. The assay was then used with 237 nasopharyngeal specimens from patients with pneumonia. Twenty-one of 237 (8.9%) were positive for C. pneumoniae, and 42 (17.7%) were found to inhibit the PCR. Specimens showing inhibitory activity were diluted 1:10 and were retested. Ten specimens were still inhibitory to the PCR and required further DNA purification. No additional positive samples were detected and 3 nasopharyngeal specimens remained inhibitory to PCR. Coamplification of a HIC DNA can help confirm true-negative PCR results by ruling out the presence of inhibitors of DNA amplification.