Previous studies on the basis for the immunosuppressive potential of adrenal corticosteroids have stressed that the effects of these agents on immune functions depend on the animal species being considered, as well as the subpopulations of lymphocytes involved in the expression of immune functions examined. In the present work, we have evaluated the effect of a single dose of hydrocortisone on three different immunoregulatory functions that can influence the magnitude of an antibody response to Type III pneumococcal polysaccharide (SSS-III) in mice; these functions include suppressor, amplifier, and helper activity that are dependent upon the presence of distinct subpopulations of thymus-derived (T) cells. The results obtained show that a single injection of a relatively large dose of hydrocortisone, when given at the time of priming with carrier, eliminated all evidence of carrier-specific helper T cell activity; hydrocortisone was also found to eliminate a significant amount of helper T cell activity when given after such activity had been generated. But, under the same experimental conditions, suppressor and amplifier T cell activities were unaffected, even in this steroid-sensitive species. Such selective sensitivity may account for some of the immunosuppressive potency of steroids.