The paper reports the distribution of lymphocyte subpopulations in the Caucasian population of Romania. Investigations were carried out in cells bearing the following antigens: CD3 (T cells), CD19 (B cells), CD4 (T helper/inducer cells), CD8 (T suppressor/cytotoxic and some NK cells), and CD16 CD56 (NK cells). Reference values for the lymphocyte subpopulation were obtained from over 100 healthy Caucasian adult volunteers. Blood from these donors was analyzed using FACScan flow cytometer, Leuco-GATE, Simultest and FACS Lysing Solution, and SimulSET software. As an internal quality control, it was verified that %T+%B+%NK approximates 100% in all samples. The results presented here, obtained on healthy donors (51 males and 49 females), showed that there are no statistically significant variations of CD4/CD8 ratio related to sex or age, the mean value of this ratio (2.0 +/- 0.02) being similar to that reported by the West-European countries. Additional similarities were found when the relative percentage, mean +/- standard deviation (CD3 = 74.2 +/- 2.8; CD19 = 10.8 +/- 1.6; CD4 = 42.0 +/- 2.5; CD8 = 28.9 +/- 5.7; CD56 = 15.2 +/- 0.4) and the absolute cell number of the major peripheral blood mononuclear subsets established in our study were compared with other published results. This study was entirely supported by Becton Dickinson--Europe (Division Heidelberg, Germany).