Abstract: Based on the development of a cloning assay for humanT-lymphocytes and leukaemic cell lines at limiting dilution in microculture, we extended the assay system to include the assessment of sensitivity to antitumour agent.A simple, sensitive and accurate technique has been established using this system. The accuracy can be calculated in the range of approximately 10^-5 to 10^-6. Comparing the estimation of viable cell count and tritiated thymidine incorporation, this assay appears to be more sensitive and the result reflects the effect of killing tumour stem cells.