The change occurring at the time when antigen it acting on the peritoneal monocytes in the rabbits sensitized with bovine serum was observed under an electronmicroscope. By the action of antigen monocytes become swollen and devoid of pseudopodia, at the same time mitochondria also swell up and cristae ase destroyed. Likewise endoplasmic reticulum swelling up, grow indistinct. Moreover, when antigen is made to act on the supra-vitally stained cells, in the case of Neutral Red there is no significant difference in the cell picture from that of cells receiving no supra-vital staining. As has previously been clarified, since Neutral Red is the substance existing in the cell as the dye droplets showing no affinity to the any one of cell organellae, the dispersion of dye particles seem to be due to the destruction of endoplasmic reticulum, which is supposed to keep the dye as droplets. Janus Green has been proved to have a special affinity to mitochondria, and in the cells previously stained supra-vitally when they come in contact with antigen, their mitochondria do not expand as much as in the case of unstained cells, but the cells show a marked agglomeration of cytoplasm. This seems to be due to the concomitant acceleration in the affinity of Janus Green to organellae other than mitochondria, and also due to the changes in molecular arrangement in the cell at the time of the swelling of endoplasmic reticulum.