We carried out an immunohistological and morphological study on Pneumocystis carinii originating from the common shrew, Sorex araneus. Immunologic properties were studied by applying two commercially available immunofluorescence staining kits with differing developmental form specificity to a lung homogenate. The cyst form-specific staining kit reacted with cysts originating from S. araneus. Ultrastructurally this particular antigen epitope specifically deposited on the electron-lucent middle layer of the cyst pellicle. The immunohistochemical staining kit reacting with both cyst and trophozoite forms from human and rat origin did not react with any developmental forms of P. carinii originating from S. araneus. Both kits demonstrated P. carinii in the lung homogenate of a field vole, Microtus agrestis. In morphologic examination, the methenamine silver-stained cyst forms of P. carinii from S. araneus and from M. agrestis differed in size from each other and from those originating from laboratory rats. Ultrastructurally P. carinii from S. araneus did not differ from organisms of rat origin.