1. Leukocyte homogenates show active, reproducible respiratory activity in a fortified system.
2. Chloramphenicol inhibits leukocyte respiration in vitro when used in concentrations similar to other commonly used inhibitors.
3. Chlortetracycline, oxytetracycline, penicillin and streptomycin have no significant effect on leukocyte respiration.
4. Urethane and atabrine produce inhibition of leukocyte respiration, but the latter is more active.
5. The possible sites of action of chloramphenicol in cellular metabolism are discussed.