The in vitro effect of glucocorticoids (GCs) on IL-1-R expression of human PBMCs was investigated. Both physiological and pharmacological concentration ranges of GC increased the specific binding of 125I-labeled human rIL-1 alpha to PBMCs. This enhancement was specific for GC, since other steroid hormones, such as progesterone, 17 beta-estradiol, and testosterone failed to elevate the binding of 125I-IL-1 alpha to PBMCs. The effect was time dependent with maximal effect occurring 6 h after treatment and dose dependent with half-maximal effect elicited by 100 nM prednisolone. Scatchard plot analysis indicated that 125I-IL-1 alpha binding increased from approximately 100 IL-1-R per cell to 2 X 10(3) receptors per cell without a major change in affinity (Kd = 2.6 X 10(-10) M). The subpopulation of PBMCs induced by GC to express higher levels of IL-1-R consisted predominantly of B lymphocytes, but not T lymphocytes, large granular lymphocytes, or monocytes. GCs also induced the expression of IL-1-R on some other cell types, including normal human dermal fibroblasts and the human large granular lymphocyte cell line YT. Since cycloheximide and actinomycin D inhibited the induction of IL-1-R by GC, synthesis of both new RNA and protein seems to be required for IL-1-R induction. This study presents the first evidence of upregulation of the receptors for IL-1 by GC, and may account for the reported enhancement of in vitro and in vivo humoral immune responses by GCs.