The effects of filtration on the inactivation of vegetative cell suspensions (Escherichia coli and Salmonella typhimurium) and spore suspensions (Bacillus subtilis and Bacillus licheniformis) by hydrostatic pressure treatment were investigated. There was no significant difference in the inactivation ratios between the filtrated and unfiltrated vegetative cell suspensions. However, filtrated spore suspensions were inactivated more easily than the unfiltrated ones. B. subtilis spores filtrated were sterilized (6-log-cycles) in a 90 min treatment, but the unfiltrated spores were inactivated 5-log-cycles in a 180 min treatment. B. licheniformis spores filtrated were inactivated approximately 4-log-cycles, but the unfiltrated spores were inactivated approximately 3-log-cycles in a 180 min treatment. These results indicated that the filtration of spore suspensions was effective to increase the inactivation ratio by the hydrostatic pressure treatment. However, filtration did not contribute to increasing the inactivation ratio of vegetative cell suspensions.