Commingling analysis of total leukocyte, neutrophil and lymphocyte concentrations in human blood suggested the effect of major regulatory factors affecting each type of white blood cells. Our results showed two distributions for the total leukocyte and lymphocyte concentrations. The values of the low and high phenotypes were 5.8 and 8.4 × 10⁹/l of total leukocytes and 2.0 and 4.6 × 10⁹/l of total lymphocytes. On the other hand, the neutrophil concentration in blood is probably regulated by equally penetrant factors. The means of the three phenotypes were 2.6, 4.2 and 6.5 × 10⁹/l. The evidence of two or three phenotypes, although consistent with the effect of a single-factor hypothesis (genetic or environmental), must be confirmed by segregation analysis of families.