In this paper the author reports how the morphologic and histologic pictures seen by the tuberculous infection can be modified by the coexisting lysolecithin. Observations revealed that the guinea pigs infected with tubercle bacillus and treated with peritoneal or intravenous injection of lysolecithin daily caused a severe tissue damage, especially with the marked caseous degeneration of lymph nodes and spleen. The intra-pulmonary injection of the bacillus mixed with lysolecithin dissolved in liquid paraffin resulted in the formation of the caseous pneumonia with tuberculous cavity. From these observations it can be said that the lysolecithin produced in the tissues by tuberculous infection plays a great role in the development of the tuberculous tissue change, leading to the infiltrative and caseous changes. The morphologic changes in human tuberculosis varying at different stages have been discussed from the view point of lysolecithin production in tissues.