Cytotoxic effects of high energy shock waves in different in vitro models: influence of the experimental set-up. High energy shock waves (electromagnetically generated, Siemens Lithostar) were studied for their effects in vitro on different (tumor) cell types. Cells were exposed to the shock waves as a single cell suspension or as a cell pellet on the bottom of a test tube. In both cases, a dose dependent direct cytotoxicity, established by trypan blue dye exclusion, was observed after treatment with 1000 or 2000 shock waves. Also, the antiproliferative capacity as determined by clonogenic potential (double layer soft agar) and growth rate (plastic) were affected in this way. However, comparing the results after treatment in suspension or pellet, a discrepancy was evident. The cell lines showed a different susceptibility in pellet vs. suspension. Also the differential sensitivity of the cell types varied in these two treatment models. Furthermore the outcome depended on the cell concentration; direct cytotoxicity in a cell suspension was more pronounced at higher cell concentrations, while in a pellet this was increased by decreasing the number of cells. Finally, no shock wave induced cytotoxicity could be seen after fixation of cells in gelatine or by placing the pellet on a bottom layer of gelatine. Pressure measurements revealed no adequate explanation for this phenomenon. These results indicate that in vitro effects depend on the way cells are exposed to the shock waves and can be greatly influenced by changing the conditions of the microenvironment. Therefore, precise descriptions of the experimental set-up and careful interpretations of their outcome are obligatory. 