The authors have been interested over the last year in the detection of pneumocystis carinii in patients with immune deficiencies, whether natural or artificial. After a brief historical and general introduction, they discuss their procedures, the pathological substances available, the staining techniques and the examination of autopsy specimens. They undertook experimental work on this micro-organism, e.g. the development of the germ in immuno-depressed rats and, from this material, numerous varied trials of culture and inoculation in new-born animals, chick embryos and cell cultures. Microphotographs illustrate this work which is still in progress.