Natural and synthetic steroids and mineralocorticoids were tested for their in vitro suppressive activity on phytohemagglutinin (PHA) stimulated lymphocytes. Three different (p less than 0.05) groups were identified. Methylprednisolone and betamethasone were very potent, dexamethasone, hydrocortisone and prednisolone were of intermediate potency, and aldosterone, prednisone and the metabolites of hydrocortisone were of low potency. In general, synthetic steroids were considerably more potent than naturally occurring compounds, but the relatively low potency of dexamethasone was unexpected. These in vitro findings rank glucocorticoid potency differently from the relative anti-inflammatory activities reported in the literature.