Disclosed is a medium for cultivating tumor cells, comprising the following constituents, as comprised for example in the medium RPMI 1640: (a) fetal calf serum (FCS), (b) penicillin-streptomycin, (c) L-glutamine, (d) transferrin, (e) insulin, (f) human epidermal growth factor (EGF) and (g) .alpha.-TGF. A method for characterizing tumor cells or potential tumor cells, respectively, is also described. Said method includes the culturing of cells in the inventive medium and the two tumor cell lines LNHOS1 and PTHOS1 established by means of the inventive culturing method.