MICA/B molecules (MICs) are stress-induced molecules expressed by infected and tumor cells. Their expression also characterizes trophoblast cells. Cytotoxic lymphocytes, including natural killer (NK) cells, express the NKG2D receptor, aiding them in the recognition and destruction of target cells that present MICs. To evade destruction, target cells employ various defense mechanisms, including the secretion of soluble forms of MICs. Choriocarcinoma JEG-3 cells and NK-92 cells were used to assess the expression of MICs and NKG2D. The cytotoxicity of NK-92 cells against JEG-3 cells in the presence of trichostatin A (TSA), anti-MICA/B antibodies (anti-MICA/B), and recombinant MIC proteins (rMICA/B) was evaluated. JEG-3 cells and NK-92 cells express MICs. Additionally, NK-92 cells exhibit high levels of NKG2D receptor expression. TSA treatment reduced the surface expression of MICs on choriocarcinoma cells, and was also associated with the release of soluble MICB. However, the TSA-induced decrease in MIC expression by choriocarcinoma cells did not protect them from the cytotoxic effects of NK cells. Only the activation of NK cells by IL-12 resulted in a decline in susceptibility of TSA-treated choriocarcinoma cells to NK cell-mediated cytotoxicity. Thus, NK cells activated by IL-12 lose their ability to effectively kill TSA-treated choriocarcinoma cells through the MIC-mediated mechanisms.