The epidemiology of P. carinii infection is poorly understood. Studies based on the presence of antibodies in serum suggest that most adults have been exposed to and been infected with P. carinii pneumonia. Thus the current concept for P. carinii pneumonia is that individuals are infected when they are young and are either asymptomatic or develop inapparent disease. Following such infection the organism becomes latent and can be reactivated at a time when the individual becomes immunosuppressed. Based on this hypothesis, one should be able to detect P. carinii in the lung tissue of a large number of individuals without P. carinii pneumonia. To investigate this hypothesis we are undertaking to evaluate pulmonary specimens from a variety of sources utilizing the PCR technique. This is a technique that allows amplification of very small amounts of DNA if the sequence for such DNA is known and primers for the PCR process can be synthesized. Recently the coding sequences for ribosomal RNA and mitochondrial ribosomal RNA of P. carinii have become available. Ribosomal RNA sequences are being evaluated primarily since multiple gene copies are present in each genome. The goal of this phase of the study is to develop more sensitive non-invasive techniques for diagnosing this infection. Optimal conditions for PCR were established by preliminary studies. We then undertook to compare PCR to standard stains for diagnosing P. carinii pneumonia. We also compared 2 sets of primers, one using published sequences, and another using our own sequences in a nested, double amplification technique. Over one hundred clinical samples have been examined to date. PCR has been found to be more sensitive than standard techniques in detecting P. carinii in induced sputum specimens. The nested double amplification technique is superior for the single amplification technique in detecting P. carinii in sputum and blood specimens. Evaluation is continuing to see if P. carinii pneumonia can be diagnosed by examination of blood.