Measurements of complement conversion and white cell variations were made on sequential blood samples obtained from a single volunteer following repeated administration of Althesin. The results suggest a mechanism by which a clinically significant hypersensitivity reaction to the drug might be mediated. Studies of patients receiving routine anesthesia revealed a very high incidence of subclinical hypersensitivity reactions, some of which appear to be immune-mediated. These reactions occurred whether or not the patients were induced with Althesin, methohexitone or propanidid.