Preliminary studies have suggested that depletion of T lymphocytes from donor marrow might be an effective method for preventing acute graft-v- host disease (GVHD) after allogeneic bone marrow transplantation in humans (Lancet 1:369, 472, 1984). However, the minimum degree of T cell depletion required to assure the prevention of GVHD in a population of marrow graft recipients has not been defined, largely because quantitative assays with sufficient sensitivity for detecting small numbers of residual viable T cells have not been developed. We have investigated three methods for the detection and enumeration of T cells after treatment of bone marrow with murine monoclonal anti-T cell antibodies and complement. Cell populations prepared by adding graded numbers of T cells to treated bone marrow were analyzed by immediate indirect immunofluorescence, by indirect immunofluorescence after culture of cells in medium containing phytohemagglutinin (PHA), or by a limiting-dilution assay. Immediate indirect immunofluorescence could reliably detect 300 T cells per 10(5) treated bone marrow cells. Indirect immunofluorescence after culture in PHA was tenfold more sensitive and could reliably detect 30 T cells per 10(5) treated bone marrow cells. The limiting dilution assay could be 300-fold more sensitive than immediate indirect immunofluorescence and 30-fold more sensitive than indirect immunofluorescence after culture in PHA. Sensitive, quantitative assays will be useful in guiding the development of methods for efficient removal of T cells in donor marrow, and will be essential for monitoring and interpreting the results of clinical trials.