The objective of the research is to learn the mode of transmission and natural habitat of P. carinii. Prerequisite to these studies is the need for a susceptible P. carinii-free animal model to serve as a host under experimental challenges. We have been successful in developing such a model with CD axenic weanling germ-free rats maintained in a Hoeltge isolator and given sterile prednisone orally. Because of morphologic, tinctorial and immunologic similarities of two algae, Prototheca and Chlorella, sp. these were investigated for their potential to cause disease in animals. Both of these organisms caused deep-organ infections in immunosuppressed rats.