The development of in vitro experimental chemotherapy against P.carinii has been hindered by the lack of efficient methods for continuous cultivation of the microorganism. Various short-term systems, allowing the production of infectious forms of Pneumocystis, can be employed for in vitro experimental chemotherapy. The purpose of the present study is to describe a statistically relevant algorithmic model for the evaluation of P.carinii in short-term in vitro culture and drug screening.