Strain ST67P of Staphylococcus hyicus was capsular type I (++)/II(+) of S. epidermidis as determined by the method of Ichiman. To mice immunized with heat-killed vaccine of strain ST67P, homologous strain and strain ATCC 31432 (capsular type I), SE-360 (capsular type II) and SE-10 (capsular type III) of S. epidermidis were injected intraperitoneally into mice, then, the viable cell number of the organisms in the peritoneal cavity were enumerated of 30 minutes and 20 hours after the injection. Results showed that the viable cell number of the homologous strain and strain ATCC 31432 was remarkably decreased at 20 hours after the injection, however, the cells were increased with strain SE-360 and SE-10. Passive protective activity of rabbit anti-ATCC 31432 serum was absorbed either with homologous strain or strain ST67P in the mouse, however, protective activity of anti-SE-360 strain serum and anti-SE-10 strain serum was not absorbed with these organisms although the activity was absorbed with homologous organisms. With an ultrathin section preparation of strain ST67P conjugated with ferritin-labelled rabbit anti-homologous strain serum, numerous ferritin granules surrounding the outermost layer of large capsule were electronmicro-scopically demonstrated. In the same organisms treated with ferritin-labelled anti-ATCC31432 strain serum, the all walls were surrounded by a relatively thinner capsule and a number of ferritin granules were located in the outermost layer of the capsule. However, with the organisms treated with ferritin-labelled anti-SE-360 strain serum only a number of ferritin granules were shown on the surface of the cell walls, and neither capsule nor ferritin granules were exhibited in the organisms treated with ferritin-labelled anti-SE-10 strain serum.