Corticosteroids are widely used in the treatment of allergic and inflammatory conditions. It is important to recognize that there are great species differences in the responses to glucocorticoids and that man is a "steroid-resistant" species. Steroids affect metabolism and distribution of T and B lymphocytes, but do not significantly affect antibody production in man. Steroids profoundly affect the inflammatory response by way of vasoconstriction, decreased chemotaxis, and interference with macrophages. Steroids affect types I, III, and IV mechanisms of immunologic injury. There are still enormous gaps in our knowledge of the actions of glucocorticosteroids.