A device was made to analyze the pneumotropism of Sendai virus in mouse. Minced lung blocks were prepared from the mouse intranasally infected with Sendai virus for 2 hours and cultured in a CO2 incubator. This culture system provided a suitable in vitro model of Sendai virus infection in mice in terms of the distribution of the viral antigens and histopathological findings. The progeny virus recovered from the lung culture was already activated and was accompanied by the cleavage of F glycoprotein into F1 and F2. This fact demonstrates that the activating mechanism is reversed in the lung culture as found in vivo infection of mouse lung. The viral activation and the cleavage of F glycoprotein were simultaneously inhibited by tosyllysylchloromethylketone, leupeptin, soybean trypsin inhibitor and antipain, but not by tosylamidophenylethylchloromethyl-ketone, chymostatin, pepstatin, iodoacetamide, phenylmethylsulfonylfluoride and p-chloromercuribenzoate. These results show that the activating enzyme of Sendai virus found in the lung culture was similar to trypsin. The existence of the activating enzyme may support the replication of Sendai virus in mouse lung in multiple-step and also result in the lung pathology.