There is speculation that high cytotoxic T lymphocyte precursor frequencies (CTLpf) correlate with poor clinical outcome of bone marrow/organ transplantation. It is also believed that human umbilical cord blood is immunologically naive, and, therefore cord blood T cells may be less able to mediate graft versus host disease than marrow-derived T cells. CTLpf were determined in peripheral blood mononuclear cells collected from healthy adults, human umbilical cord blood and renal dialysis patients who were randomly selected and entered into this study. A highly sensitive non-radioactive Europium release cytotoxicity assay was optimized and modified to carry out the CTLpf estimation by using the principle of limiting dilution analysis. The results of CTLpf in healthy adults ranged from 1/694 to 1/66,666, median 1/7,339 (n=10); cord blood ranged from 1/1,562 to 1/35,714, median 1/10,162 (n=6) and dialysis patients ranged from 1/1,054 to 1/17,857 median 1/5,208 (n=9). The results demonstrated that there is little difference of CTLpf median values between the groups, but there is a wide variation of CTLpf between individuals within a population. It suggests that this variation should be taken into account when considering CTLpf assay as pre-transplantation cross-match procedure.