In order to clarify the mechanism of the formation of epithelioid cells (“EG”) some morphological experiments have been done using the rabbits inoculated with several fractions of tubercle bacilli. By fractionation and purification of Mycobacterium tuberculosis H37RV, phosphatide, wax‐A, wax‐B, wax‐C, wax‐D and residual component were obtained. Albino rabbits were inoculated with these tubercle bacillary constituents into the lymph nodes and light and electron microscopic observations of popliteal lymph nodes have been made. EC could be classified into three groups and their subgroups in accordance with the ultrastructural features of the phagocytes and of digestion bodies contained in them. Thus “the pre‐ or non‐typical EG” seemed to be derived chiefly from the preexisting mature phagocytes and “the digestosome type EC” from immature phagocytes. “The phago‐digestosome type EC” had the intermediate characteristics. Among many mycobacterial fractions only phosphatide and wax‐D had the ability to form EC.In comparison with the case of non‐ sensitized animals, emergence of EC was generally more prompt and massive in animals which had been slightly or moderately sensi