Platinum-resistance is the most crucial problem for treatment of ovarian cancer. There is a clinical need for new treatment strategies which overcome platinum resistance. As survival is strongly influenced by immunological parameters, immunotherapeutic strategies appear promising. Therefore a better understanding of the interaction between ovarian tumour cells and cells of the immune system is a necessary prerequisite. In the present study we aimed to enlighten the interactions between platinum resistant and platinum sensitive ovarian cancer cells and natural-killer (NK)-cells. Modified FATAL assay was used for determining the killing efficiency of NK-cells for the parental A2780 cells and the cis-platinum resistant A2780cis human ovarian cancer cells. Expression of pro- and anti-apoptotic genes as well as ligands involved in NK-cell receptor recognition were analysed by RT-PCR and flow cytometric analysis. The efficiency of NK mediated cell lysis differs between A2780 cells and the cis-platinum-resistant A2780cis cells. A2780cis cells are less accessible for NK-cell mediated killing. Based on this observation we characterized the molecular basis for resistance mechanisms. Besides an increase in anti-apoptotic genes (especially CIAP-1 and -2) that probably render A2780cis cells more resistant against apoptosis an increased amount of soluble MICA/B seems to be responsible for the lower killing rate of platinum-resistant A2780cis cells compared to their parental A2780 cells.