Electron microscopic observations of Streptococcus pyogenes (strain S43, type 6) treated with trypsin and cetyltrimethylammonium bromide (Cetab) are presented. The concentration of trypsin necessary to remove the M protein antigen was shown to bring about a partial digestion of the cells. The cell protoplasm and protoplasmic membrane were affected. No microscopic changes appeared in the cell wall. Cetab did not alter the appearance of the cells. Both trypsin and detergent altered the permeability of the cell so that citrulline and carbamylphosphate were metabolized. Cells exhibiting enzyme activity on these substrates after Cetab treatment were characterized by the release of lysine, glutamic acid, nucleic acid, and other cellular material. Enzymes responsible for the metabolism of arginine, citrulline, and carbamylphosphate were shown to reside in the cell protoplasm. Cells which had lost their viability after Cetab treatment still possessed the ability to utilize the latter substrates. Under similar conditions the metabolism of glucose did not occur. Normal cells were shown to possess the ability to fluoresce in the presence of a dye. The intensity of the fluorescence was reduced by trypsin treatment.