Wistar rats were induced of Pneumocystis carinii infection by injection with methyl-prednisolone to correlate the cyst counts and numbers of nuclei. Seven sections of the lungs were examined by impression smears and also whole lung homogenates were screened for nucleus counting for each rat. At the first week of the experiment, all of the impression smears except one were cyst negative but trophic forms were counted around 10(6). At the third week, the cysts appeared one per 20 immersion oil lens fields. The nuclei were on the order of 10(7) at this period, and this amount of Pc is regarded as the limit of cyst detection on impression smears. When the nuclei were over 10(9) in the lungs, the cysts were counted about 50 in 20 microscopic fields. The organisms were distributed in the lungs without any predilection focus. The present data suggest that the trophic forms, proliferate first and the cysts appear later in the lungs.
