Murine and human studies have demonstrated that the normal liver contains significant numbers of resident lymphocytes that have functions distinct from those found in blood and other organs. To characterize these cells requires the isolation of viable lymphocytes that can be analysed by flow cytometry and in functional assays. The techniques classically used to isolate single cell suspensions of hepatic lymphocytes for phenotypic and functional studies involve mechanical and/or enzymatic dissociation of liver tissue. The aim of this study was to determine the effect of these procedures on surface molecule expression and lymphocyte function and to optimise an isolation technique that minimises these effects. Mechanical homogenisation of liver tissue alone resulted in low viable lymphocyte yields but these were improved by the combined use of mechanical and enzymatic techniques. A mean yield of 2.3 x 10(6) lymphocytes with a mean viability was 88.8% was obtained from 200 mg wedge biopsy samples of normal adult human liver using a combination of gentle mechanical dissociation followed by digestion with collagenase type IV and DNase I. These cells were suitable for phenotypic characterisation by flow cytometry. They also retained their ability to grow in vitro, to respond to cytokines and activation stimuli, to mediate cytotoxic killing of target cells, and to produce inflammatory and regulatory cytokines.