A method for the determination of the concentration of zinc in a well-characterized fraction of mononuclear leucocytes from human blood is described. Leucocytes were separated from whole blood by use of the one-step sodium metrizoate/Ficoll procedure. The cell suspensions obtained by the separation procedure had the following composition (mean, range) (N = 15): 82% (70-85%) lymphocytes, 15% (10-15%) monocytes and 4% (2-10%) neutrophils. The ratio of platelets to leucocytes was 1:1 (1:1-1.5). Cell pellets were sonicated before zinc analysis by flame atomic absorption spectrophotometry. 10 healthy volunteers (5 males and 5 females) aged 21-45 years, median age 33 years, were studied. The concentration of zinc in mononuclear leucocytes was 1.14 +/- 0.14 mumol/10(10) cells or 156 +/- 8 mumol/g protein (mean +/- SD).