Correlations between therapeutic response of leukaemias and in-vitro drug-sensitivity assay. To develop the differential staining cytotoxicity (DiSC) assay, an in-vitro drug sensitivity test designed specifically for use with fresh human haematological tumour cells, into a predictive test for response to therapy as well as a tool for identifying new treatment strategies, test results have been correlated with response in patients with haematological malignancies. 22 of 119 tests indicated extreme drug resistance (tumour cell survival greater than 55%) in vitro. None of these patients responded to chemotherapy. The proportion of patients responding to therapy and 50% patient survival rose with in-vitro drug sensitivity. 11 specimens showing extreme drug resistance to the agents prescribed for the patient were subjected to drug sensitivity tests of drugs not prescribed; patients showing some sensitivity to these drugs had better survival than did those with extreme drug resistance to all drugs tested. The DiSC assay can also be used to identify new agents worth testing in clinical trials, and cross resistance profiles of new agents may indicate what combinations might be useful. 