Bisphosphonates are potent inhibitors of bone resorption in vivo and are emerging as important and widely used drugs for the treatment of a variety of abnormal bone resorptive processes. In the current study we investigated the in vitro effects of 4-amino-1-hydroxybutylidene-1,1-bisphosphonate (alendronate), a recently developed, extremely potent bisphosphonate, on the immune functions of human peripheral blood mononuclear cells (PBMCs). PBMC proliferation induced by lectins, alloantigens, and a nominal antigen (tetanus toxoid) was inhibited in a dose-dependent manner by alendronate. Pretreatment of monocytes, but not T cells, with the compound at concentrations ranging from 10(-4) to 10(-8) M was inhibitory, indicating that alendronate acts selectively on antigen-presenting cells (APCs). Alendronate did not affect the viability of monocytes or T cells or the expression of cell surface molecules known to play critical roles in antigen presentation. Alendronate exhibited dose-dependent inhibition of the production of interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha) by activated monocytes. The inhibitory effect of 10(-6) M alendronate on PBMC proliferation was reversed by 10 U/ml recombinant rIL-1 beta, whereas other cytokines such as IL-6, TNF-alpha, and granulocyte-macrophage colony-stimulating factor (GM-CSF) had no effect. Thus, alendronate acts on monocytes to inhibit their antigen-presenting/accessory cell functions through a mechanism that can be overcome by exogenous IL-1. The inhibitory effect of this agent on cytokine production may contribute to its inhibitory effect on bone resorption.