Non-specific interstitial pneumonia (NIP) occurs frequently in patients with HIV-infection. To elucidate the etiology of this pulmonary disorder, we searched for 13 different microorganisms in transbronchial biopsies from 15 patients with NIP, 15 patients with Pneumocystis carinii pneumonia (PCP) and 20 patients with lung diseases not related to HIV-infection using monoclonal antibodies and the APAAP- or PAP-technique for immunostaining. Chlamydia trachomatis and parainfluenza III were detected frequently and in great number. Adenovirus, influenza B, varicella zoster and cytomegalovirus were also found frequently, but not in great number. Measles virus, respiratory syncytial virus, influenza A and herpesviruses 1&2 were not found. Also not found were parainfluenza I, mycoplasma pneumoniae and coronavirus. In seven out of fifteen NIP patients at least one organism was shown, compared to nine out of fifteen patients with PCP and eight out of twenty patients in the control group.