As part of a study to define the factors affecting the distribution of lymphocyte subpopulations in healthy blood donors, we have measured lymphocyte surface markers in various racial groups. Markers tested were T3 (all T cells), T4A (T helper cells), T8 (T suppressor cells), and Leu 11 (natural killer cells). Racial groups included three Asian groups (Chinese, Japanese, Other Orientals) and three non-Asian groups (Caucasians, Hispanics, American Blacks). The mean percentage of T3 + cells and T4A + cells were significantly lower in Asians compared with non-Asians, while T8 levels did not differ. These changes resulted in a significantly lower mean T4A:T8 ratio in Asians compared with non-Asians. The mean percentage of Leu 11 + cells was higher in Asians compared with non-Asians. Within the Asian group, Chinese had a higher mean Leu 11 + value than the other two Asian groups combined. Further, pairwise comparisons showed that Chinese had a significantly higher mean Leu 11 + value compared with each of the other five racial groups. This increased mean Leu 11 + level in the Chinese group reflected a distinct cluster of high values for about half the subjects. These findings show that racial background should be a major consideration when defining the normal range for lymphocyte subpopulations detected by surface markers.