Determination of chemosensitivity/chemoresistance is becoming increasingly important for individualization of breast cancer chemotherapy. We developed a simple non-destructive test of cellular activity (NTCA) for assessment of the cytopathic effect of antitumour drugs in vitro. Contrary to routinely used methods (e.g. MTT), besides the comparative evaluation of metabolic activity using pH (given by the medium colour), the NTCA enables the simultaneous assessment of proliferation and morphology of cultured cells (phase-contrast microscopy) at any time during the incubation with cytostatics. Moreover, the regenerative potential of the cells can be examined by cell recovery and growth after drug removal. We provide evidence for the relevance of NTCA in chemosensitivity testing of primary breast cancer cells and breast cancer cell lines for cisplatin, gemcitabine and tamoxifen. NTCA represents a simple addition to the chemosensitivity assessment and could also serve for rapid screening of new antitumour strategies.