By studying the vital findings, vital stainings with neutral red and Janus green, the intake of lithium carmine dye, and carbonparticle phagocytosis, in the fibroblasts and histiocytes appearing in the bone-marrow tissue culture of healthy rabbit femur by coverslip method, and also by observing the fixed and stained specimens of cultured tissues, the author studied the morphological and functional properties of these cells and also scrutinized the relationship of these cells with monocytes and the possiible mutual transformation. 1. Fiqroblasts are spindle-shaped, possessing peculiar granules but showing no movement or transformation at all; and they contain only a few neutral-red granules, taking no lithium carmine dye and having practically no phagocytic capacity. 2. Histiocytes are polymorphous, and those oval shaped ones have tentalce-like projections around their cell-body, which are extending and contracting. Although their motility is not so marked, their morphological changes can be observed. Histocytes on the other hand can take in lithium carmine dye and their phagocytic capacity is marked, showing a deep neutralred stain as well as a few granules stained by Janus green. 3. In fibroblasts, histiocytes and monocytes can be observed the characteristic shape, movement and function of respective cells, but there can be recognized no mutual transformation among themselves in the observations up to the tenth day of the culture.