In vitro studies designed to examine the mechanisms of immune tolerance after liver transplantation in humans have been hampered by the difficulty in obtaining sufficient numbers of donor liver-associated leukocytes (LALs). We have investigated whether the ex vivo perfusion of donor livers releases a population of LALs that can be readily retrieved from the waste fluid. The mean number of cells recovered after Ficoll-Hypaque density-gradient separation was 2.6 +/- 0.5 x 10(8) cells, with a viability of 94% +/- 2%. The perfusate lymphocytes comprised mainly T cells (39% +/- 2%) with a very low CD4/CD8 ratio and natural killer (NK) cells (56% +/- 6%) with an increase in the proportion of the CD3-CD56+CD16- subset. The activation marker CD69 was present on the majority of the perfusate lymphocytes. These are the phenotypic characteristics that have been previously reported for lymphocytes isolated from hepatic sinusoids. In mixed lymphocyte reactions, the perfusate cells showed a marked increase in the ability to stimulate allogeneic responder cells, resulting in 353% +/- 78% (P = .003) greater incorporation of [3H]thymidine in responder cells when compared with stimulation by donor peripheral blood mononuclear cells. The results show that large numbers of viable donor lymphocytes can be readily isolated from the liver perfusate solution. These cells have the characteristics of liver-associated lymphocytes with a predominance of activated NK and CD8+ T cells. This population can now be used in in vitro assays to elucidate the influence of donor leukocytes on the development of graft acceptance.