A method for separating clusters of malignant cells and clusters of stromal cells of a malignant tumor tissue sample ex vivo, characterized by manner and order of the following steps: (a) the malignant tumor tissue sample is purified and minced, preferably in pieces of about 1-4 mm 3 (cubic millimetre) volume, (b) the minced tissue is suspended in culture medium, (c) the suspension obtained in (b) is subjected to an exclusive treatment solely with collagenase (i.e., a collagenase digestion or a collagenase decomposition, respectively) for a partial collagenase digestion, wherein Clostridium histolyticum collagenase is used in a concentration of 200 CDU to 250 CDU per mL at approximately 36°C to 37°C for a period of approximately 4 to 5 hours, (d) the collagenase digestion product obtained in (c) is centrifuged, preferably for about 5 minutes at 50 x g and room temperature, (e) the pellet obtained in (d) is separated from the supernatant, (f) the pellet of the collagenase digestion product from (e) is decomposed and resuspended in culture medium and thereby aspirated and returned again and sucked in again for about 4-6 times by means of an aspiration device, preferably a pipette tip, thereby subjected to a treatment with hydrodynamic shear forces, and next transferred in a culture vessel, (g) the resuspension obtained in (f) is analysed with regard to at least one cell cluster with the help of a microscope (preferably an inverse microscope equipped with a phase lens system), wherein this cell cluster can be identified either on the basis of its phenotypic appearance as a cluster of malignant cells or on the basis of its phenotypic appearance as a cluster of endothelial cells or on the basis of its phenotypic appearance as a cluster of fibroblasts, and (h) the cell clusters identified in (g) are separated. (a) the malignant tumor tissue sample is purified and minced, preferably in pieces of about 1-4 mm 3 (cubic millimetre) volume, (b) the minced tissue is suspended in culture medium, (c) the suspension obtained in (b) is subjected to an exclusive treatment solely with collagenase (i.e., a collagenase digestion or a collagenase decomposition, respectively) for a partial collagenase digestion, wherein Clostridium histolyticum collagenase is used in a concentration of 200 CDU to 250 CDU per mL at approximately 36°C to 37°C for a period of approximately 4 to 5 hours, (d) the collagenase digestion product obtained in (c) is centrifuged, preferably for about 5 minutes at 50 x g and room temperature, (e) the pellet obtained in (d) is separated from the supernatant, (f) the pellet of the collagenase digestion product from (e) is decomposed and resuspended in culture medium and thereby aspirated and returned again and sucked in again for about 4-6 times by means of an aspiration device, preferably a pipette tip, thereby subjected to a treatment with hydrodynamic shear forces, and next transferred in a culture vessel, (g) the resuspension obtained in (f) is analysed with regard to at least one cell cluster with the help of a microscope (preferably an inverse microscope equipped with a phase lens system), wherein this cell cluster can be identified either on the basis of its phenotypic appearance as a cluster of malignant cells or on the basis of its phenotypic appearance as a cluster of endothelial cells or on the basis of its phenotypic appearance as a cluster of fibroblasts, and (h) the cell clusters identified in (g) are separated.