Recently several authors have succeeded in demonstrating pneumocysts in rat lung by electron microscopic methods (Pliess and Seifert,6 Seifert and Pliess,8 Schulz,9 Wessel and Ricken12). Only a few have reported electron microscopic investigations in human lungs with interstitial plasma-cell pneumonia: Diekmann, Lindner and Stopka4 studied suspensions of pneumocysts after centrifugation but failed to observe cyst forms similar to those seen in the light microscope. Timmel could not detect typical cysts in ultrathin lung sections. He found only thin-walled forms as well as small particles which he suggests to be virus. Seifert reported thin-walled forms too but also found one thick-walled cyst with 3 cyst bodies, each of which was partially surrounded by a double membrane.
While work with human lung tissue involves a delay between death and postmortem examination—in contrast to rat lung tissue which can be embedded immediately after death—we have found that pneumocysts