Studies were reviewed on attempts to apply serologic methods for the laboratoary diagnosis of pneumocystis pneumonia and for epidemiologic surveillance in outbreaks of this disease. At autopsy the parasite was found in 7.7% of patients with a clinically latent infection. In this series of patients, parasitologic findings were not correlated with the results of a complement fixation (CF) test or with those of an indirect fluorescent antibody test. The high incidence of serologic reactivity in the CF test in chronic obstructive lung disease was not related with special clinical symptoms. Longitudinally performed parasitologic and serologic observations will be needed to find out if Pneumocystis carinii is a pathogen and fulfills Koch's criteria. Preliminary data are given for such an experimental approach in a colony of rats.