We have studied possible sources of resting human lymphocyte DNA content variation among euploid males and females as determined by flow-microfluorometry. A major source of variation arises from instrumental instability, most notably flow-rate changes. This variation can be detected and minimized by the use of chicken erythrocytes as an internal standard. A second source of variation arises from differences in rates of lymphocyte stain uptake. The lowest coefficients of variation (0.3 %) among individuals of like sex, and optimal distinction between sexes, were achieved after exposing lymphocyte samples for 24 h to the hypotonic staining solution. Four males and females with either low or high relative DNA contents within their respective sexes showed correspondingly divergent values upon repeat venipuncture. Some of the observed interindividual DNA content variation might therefore reflect truly genetic differences. For two male samples with repeatedly divergent DNA content values, preliminary C-banding of conventional metaphases suggests corresponding differences in constitutive heterochromatin.