Methyl 20 beta-dihydroprednisolonate, a new local antiinflammatory steroid, and its ester hydrolysis product, 20 beta-dihydroprednisolonic acid, were evaluated for potential immunosuppressive actions as determined by the splenic plaque-forming cell response assay. The parent compound, prednisolone, was used as a comparative agent. All three corticosteroid agents, when administered intraperitoneally or subcutaneously by separate injections given at the same time as antigen, demonstrated significant dose-related suppression of this immunologic parameter. In a croton oil-induced inflammation test (ear challenge in mice), methyl 20 beta-dihydroprednisolonate demonstrated significant antiinflammatory effects, but only in the ear to which it was applied. The comparative drug, prednisolone, significantly reduced inflammation in the ear to which it was applied and in the contralateral ear as well. The methyl ester derivative demonstrated effective antiinflammatory activity when applied topically in the oxazolone delayed-type hypersensitivity test (ear challenge in mice). The results seen in the plaque-forming cell response are in contrast to the generally noted lack of systemic glucocorticoid actions of the steroid acid ester derivatives reported in other studies.