Pneumocystis carinii is an opportunistic pathogen causing life threatening pneumonia (PCP) in immunosuppressed patients and particularly among AIDS patients. Whether the infection results from reactivation or reinfection is debated. Since methods for in vitro cultivation still are not successful, the diagnosis is dependent on direct demonstration of the organism in respiratory specimen. In this thesis laboratory diagnostic methods in terms of staining, sampling, antibody detection and DNA amplification are evaluated. Furthermore, the occurrence of the organism in the Western world versus Africa, and in symptomatic and asymptomatic HIV infected patients is studied and discussed. The use of a monoclonal antibody (MAb), 3F6 in an indirect immunofluorescence assay (IFL) was compared to the two most commonly used chemical stains, silver methenamine and toluidine blue. The IFL method detected both cyst and trophozoite stages of P. carinii and was more sensitive than the chemical stains when applied to sputum samples. Among commercialised MAbs for P. carinii detection by IFL, only the indirect tests were readily applicable to ethanol treated HIV inactivated samples. In contrast to the MAb 3F6, (Dakopatts), the MAb from Northumbria stained only a selection of the cysts and no trophozoites. The relative sensitivity of IFL in detecting the organism in sputum samples compared to bronchoalveolar lavage (BAL) samples was estimated to be at least 70%. The polymerase chain reaction (PCR), which can amplify specific DNA fragments, was used for the demonstration of P. carinii in sputum and BAL specimens. The PCR was shown to be specific and more sensitive than IFL. However, P. carinii DNA was found in a few patients without clinical evidence of present, past or future PCP. Thus the possibility of PCR to detect colonization must be considered. Detection of antibodies to P. carinii by indirect IFL was studied in HIV versus non-HIV patients. A titer rise was seen in 45% of non-HIV patients versus only 3% in HIV patients during a PCP episode. No humoral response was seen in AIDS patients, whereas the serology did support the clinical PCP diagnosis in a proportion of the otherwise immunosuppressed patients. Serology may however not be of help in the acute setting. In the beginning of 1988 PCP had not yet been reported from Central Africa where the AIDS epidemic by then was growing fast. The occurrence of P. carinii in Central Africa was evaluated by a comparative study on induced sputum samples from HIV infected patients with pulmonary infection in Stockholm, Sweden and Lusaka, Zambia.(ABSTRACT TRUNCATED AT 400 WORDS)