The kinetics of terminal deoxynucleotidyl transferase‐positive cells in the mixed colony assay

Human bone marrow was treated with cytolytic monoclonal antibodies BA‐1, BA‐2 and BA‐3 and examined for terminal deoxynucleotidyl transferase‐positive (TdT₊) cells during culture in the mixed colony assay. The kinetics of TdT₊ cells was compared with untreated bone marrow controls over a 14‐day culture period. In control cultures a progressive decline in the number of TdT₊ cells occurred during the first three days and by day 4 no TdT₊ cells were observed. In antibody‐treated cell cultures in which all or the majority of TdT₊ cells were removed, no TdT₊ cells were detectable after 24 h. Both control and antibody‐treated cultures showed comparable colony formation from myeloid, erythroid and multipotential progenitor cells by day 14. The results therefore suggest that the mixed colony assay does not support the growth of TdT₊ cells or the production of TdT₊ cells from TdT₋ precursors.