Although Pneumocystis continuous culture systems have not yet been developed, efficient short-term in vitro methods allowing the production of infectious forms of Pneumocystis can now be employed. The quality of the inoculum will influence the in vitro development of P. carinii. For this reason, efficient extraction and cryopreservation techniques are considered in this section. In vitro growth and limited passage were obtained by inoculating freshly extracted parasites onto fibroblast- or epithelial-like cell monolayers cultivated in ordinary tissue culture flasks, culture plates, microcarrier beads or other culture devices. Cultures were usually maintained in an atmosphere of 5% CO2 at 35-37 degrees C. The results obtained in these different systems were surprisingly similar: the number of parasites increased about 6-10 times within the first 3-4 days post-inoculation, then remained stationary until day 7-14 and decreased rapidly. If passages were attempted, the growth decreased gradually and no growth was recorded after 2-3 passages. Proof of the in vitro Pneumocystis attachment to feeder cells has been furnished by electron microscopy. Two currently used feeder cell culture systems were selected in this subchapter. The first system is a co-culture of monolayer lung epithelial-like cells with Pneumocystis. After trypsin treatment and passage of cells with attached parasites to culture bottles containing fresh medium, 3 or more new culture bottles can be plated. A 2-4-fold increase in parasite number can be obtained but, interestingly, cultured parasites were more infectious to the nude rat than freshly extracted lung parasites. In the second system, the spinner flask culture method, Pneumocystis is cultivated on cell coated microbeads in slow stirring vessels, in order to exploit the beads' huge surface where microorganisms can transiently adhere and grow and from where they can be easily detached by simply leaving the beads to settle down. This culture system has ensured 10(8)-10(9) viable trophozoites in each harvest after 7-10 days of slow stirring incubation.