Microwell culture assays, termed the “negative” well technique and the “positive” well technique have been used to measure the concentration of murine haemopoietic progenitor cells (HPC) by limiting dilution. Using morphologic examination and ~3H-TdR uptake, the number of HPC of Balb/c mice per 10~5 bone marrow cells was 33 and 31 respectively. The linear relation between the number of harvested cells and input ceils seen on the 4th and 7th days of culture reflected the growth of HPC in microwells Microwell culture assays, termed the “negative ” well technique and the “positive ” well technique have been used to measure the concentration of murine haemopoietic progenitor cells (HPC) by limiting dilution. Using morphologic examination and ~ 3H-TdR uptake , the number of HPC of Balb / c mice per 10 ~ 5 bone marrow cells was 33 and 31 respectively. The linear relation between the number of harvested cells and input ceils seen on the 4th and 7th days of culture reflected the growth of HPC in microwells