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Flow cytometric DNA analysis is an attractive alternative to conventional cytogenetics for diagnosing karyotype changes resulting in abnormal DNA content. We have developed methods for long‐term storage of samples and standards, for staining the cells and for standardization of the measurements by two internal standards. In this paper the currently attainable resolution was determined and limiting factors were identified. Normal reference values for male and female leucocytes were determined by analyzing 240 samples from six men and six women, all cytogenetically normal. The DNA content of female cells was 1.5% higher than that of male cells. This finding was used to correct the results for sex related DNA differences. The 95% confidence limits for mononuclear blood cells were ±0.79%. The results on granulocytes were exceptionally variable, with 95% confidence limits of ±1.79%. Reexamination of the …