To determine the status of diphtheria immunity among our patients, we measured diphtheria antitoxin levels in three separate patient populations. The technique used was toxin neutralization in rabbit skin, which allowed us to evaluate the immune status of individual patients. We studied an emergency room population, a sampling of inpatients from an urban teaching hospital, and a group of older patients from a chronic care hospital. Overall, approximately 80% of patients had adequate diphtheria antitoxin levels in their serum. However, two subgroups emerged with lower levels, with potential nonimmunity. These were certain young patients, either foreign born or with a potentially "immunocompromising disorder," and the elderly chronically ill. We conclude that while overall diphtheria immunity in our patients appears adequate, subgroups with increased risks do exist and immunization for these subgroups should be undertaken.