The existence of a marginal lymphocyte population in rat liver sinusoids has already been demonstrated using the sinusoidal lavage method. We used the same technique to study the lymphocyte population in human liver obtained ex vivo after partial hepatectomy for benign or malignant tumors and compared it with peripheral and portal blood lymphocyte populations. Percentages of lymphocyte surface phenotypes were evaluated by flow cytometry. The lymphocyte population obtained from human liver is mainly made up of CD56+ (35%) cells. This percentage is three times greater than that found in peripheral and portal blood. Two-color flow cytometry analysis showed that within the CD56+ liver cell population, at least three distinct subsets could be found: (a) CD3+/CD56+/CD16-; (b) CD3-/CD56+/CD16-; and (c) CD3-/CD56+/CD16+. Although these subsets were also present in peripheral and portal blood, the percentage distribution was completely different because most CD56+ cells in peripheral and portal blood belonged to the CD3-/CD56+/CD16+ subset. These results show the existence of a heterogeneous natural killer cell population in human livers with tumors. The functional significance of this heterogeneity still needs to be explained.