Twenty-six cultures were examined for the MK system of aerobic gram-positive cocci. Sporosarcina ureae revealed the MK-7 system, which is consistent with that of the genus Bacillus. In the genus Planococcus, the only system composed of MK-8(MK-7) was found. The genus Staphylococcus was divided into two groups according to the MK system, viz., MK-8(MK-7) in St. aureus and MK-7 in St. epidermidis and St. saprophyticus. The genus Micrococcus was confirmed to be the most complicated; it had MK-8(MK-7). MK-8(H₂), and MK-9(H₂) in M. luteus, MK-9(H₂) in M. agilis, and MK-7(H₂) in M. varians. The pinkish red-pigmented, revised species M. agilis Ali-Cohen 1889 is distinguished from M. roseus on the basis of its MK-9(H₂) system. Discussions are made on the classification of these aerobic gram-positive cocci from the viewpoint of the MK system.