The polymorphic MICA gene encodes glycoproteins that activate T cells and NK cells through the NKG2D receptor and may costimulate immune functions. We found that MICA was expressed on freshly isolated human fibroblasts and was markedly decreased when fibroblasts were grown to confluency in culture dishes. MICA surface protein was measured by flow cytometry with the MICA-specific monoclonal antibody (mAb) 6B3, and HLA class I-specific protein was determined with mAb w6/32. In these experiments, after culture for 120 hours, the staining for MICA in fibroblasts decreased to about 20% of the initial amount and MICA mRNA fell in parallel, while HLA class I staining was maintained or even became somewhat stronger. In other experiments, MICA expression was not decreased when fibroblast contact was prevented by the addition of 1 muM Rottlerin, a specific inhibitor of protein kinase C delta known to prevent contact inhibition of fibroblasts. In the NK cell cytotoxicity assay, blocking MICA by antibody or downregulation by cell contact resulted in a decrease of specific killing by 30%. Increased MICA expression during proliferation of fibroblasts may support the host response to injury.