Pneumocystosis (Pneumocystis carinii pneumonia) is a parasitic disease which in Lithuania used to be diagnosed only with the use of pathohistological methods. The aim of our study was to determine prevalence of P. carinii antibodies among children and women, using immunochemical method (ELISA) and to find out which groups of patients have higher antibody titres. We also addressed the question of infection with P. carinii among rodents. After investigating 9 species of rodents it was found that mostly infected is brown rat (Rattus norvegicus) - 42.9% of them were found infected. The least infected was bank vole (Clethrionomys glareous)- 5.9. %. The brown rat lives on hatch of domestic birds, rabbits or drinks eggs, while rodents in nature live on insects, molluscs, amphibians etc. European water voles (Arvicola terrestris) and field voles (Microtus arvalis) are vegetable feeders. Our data shows that those species of rodents were not infected with pneumocysts.