The transport of zinc has been studied in normal human leucocytes incubated in both a tissue culture medium and Krebs buffer. 2. 65Zn influx is characterized by an initial rapid phase followed by a slower influx. 65Zn influx is directly dependent upon the extracellular zinc concentration. 3. Net zinc influx could only be demonstrated in Krebs buffer at zinc concentrations in excess of 4.3 mumol/l and in tissue culture fluid at zinc concentrations in excess of 43.1 mumol/l. 4. A 65Zn efflux rate constant of approximately 1.0 per h was observed in both 4.3 and 15.4 mumol of zinc/l of Krebs buffer or tissue culture fluid. 5. In a zero external zinc Krebs buffer the 65Zn efflux rate constant fell to 0.57 per h and was accompanied by a small net zinc efflux.
