Pneumocystis carinii pneumonia has been markedly increasing recently in Japan. Most cases are seen after receiving anti-cancer and immunosuppressive therapy against their underlying diseases. No cases have been found in undernourished or premature infants. The authors proposed a new method which can concentrate the cysts efficiently from the human and animal lungs, and express the density of infection quantitatively at the same time. This method will especially be valuable to demonstrate the organism in light infection such as latent infection, after treatment, and so on.