Antibacterial activity of ticarcillin was determined in comparison with that of sulbenicillin, amoxicillin, cefuroxime, clindamycin and metronidazole against anaerobic bacteria which have been isolated from various clinical materials in this hospital. Growth of more than 90% of Gram-negative anaerobic rod bacteria was inhibited by ticarcillin at its concentration of 100 micrograms/ml. Strains resistant to ticarcillin showed cross resistance against both sulbenicillin and amoxicillin. Antibacterial activity of ticarcillin against Gram-positive anaerobic bacteria was found almost equal to sulbenicillin but slightly inferior to amoxicillin. Cefuroxime was found most inferior among the tested six antibiotics when an inoculation level of 10(8)/ml was utilized, but it showed similar activity to ticarcillin when they were tested with 10(6)/ml inoculation. Approximately 10% of bacteroides strains was resistant to clindamycin while all the strains were sensitive to metronidazole.