Pneumocystis carinii derived from infected murine lung was found to be capable of limited growth in Vero African green monkey kidney cell cultures. The observed increase in the number of cyst forms was influenced by the ratio of cysts to host cells in the inocula, duration of passage, and formulation of the cell culture media. Maximum growth was achieved by inoculating 1.3 X 10(5) cysts per 75-cm2 flask containing about 2 X 10(7) Vero cells (cyst-to-cell ratio=1:154) maintained on minimal essential medium supplemented with 2% fetal bovine serum. Under these conditions of culture, a 10.8-fold increase in cyst forms was observed during a 7-day passage in cell culture utilizing cyst-to-cell ratios ranging from 1:28 to 1:2,778.