A reevaluation of the factors considered to affect the chromosomal sensitivity of mammalian peripheral blood lymphocytes has been made on the basis of some recent experiments using the harlequin-staining method to distinguish the first from subsequent divisions. Observations on human lymphocytes show that the mixing up of first and second or third cell cycle cells cannot explain all the variability between the experiments performed without bromodeoxyuridine labeling. Establishing a uniform methodology is also not likely to eliminate all causes of variation. Similar heterogeneity is observed for other mammalian species. Provided comparisons are restricted to results obtained within the same laboratory, some estimation of the relative sensitivity of chromosomes from different species is, however, possible.
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