The effects of single oral doses of 10, 15, or 30 mg of prednisone on circulating mononuclear cells, autologous MLR, mitogen responses, and allogeneic MLR were studied in healthy volunteers. Doses as low as 10 mg were immunosuppressive, causing diminution of circulating T cells and monocytes, and significant reduction in autologous but not allogeneic MLR responses. These effects were maximal 6 hr after drug administration and gone by 24 hr. Autologous MLR responses were particularly sensitive to the effects of prednisone being significantly and consistently suppressed 2 hr after drug administration, before significant cell redistribution had occurred. Macrophage-enriched stimulating cells were more easily suppressed than responding T cells. Since the autologous MLR may be important in in vivo regulation of immune responses, its reduction by low-dose glucocorticoids may be of clinical relevance. This suppressive effect must be considered in studies of the autologous MLR in patients receiving glucocorticoid therapy.