Problems and pitfalls in assessing human T-lymphocyte mutant frequencies. 
The measurement of 6-thioguanine-resistant frequencies in human T-lymphocytes has been used to quantitate the in vivo HPRT mutant frequency.  The data so far indicate a large variability in normal healthy individuals.  The reliability with which wells are identified for clonal growth in the assay was investigated using 5 different methods of scoring: visual scoring, uptake of [3H]thymidine (either by cut off point or by statistical analysis), cell count and cytogenetic analysis.  None of these methods presented a viable means of scoring the assay.  An examination of the statistical precision of the assay under the limitations imposed by the experimental conditions leads to the conclusion that there is a large inherent error associated with the estimated mutant frequencies.  Analysis of the T-lymphocyte subpopulations by cell surface monoclonal antibodies also leads us to believe that the observed mutant frequencies may not be representative of the true in vivo mutant frequencies.  If the assay is to be used as a sensitive screen for individual or population exposure to possible mutagens, a closer understanding of the biology of the assay is indicated, and a comprehensive reevaluation of the methodology required.  The utility of the system for studying qualitative aspects of human mutagenesis is not in doubt.