1. Both albumin and transferrin have been suggested as carriers of zinc in plasma. Their relative importance in Zn transport was therefore investigated as a preliminary to a study of the rates of passage of Zn through plasma. 2. The apparent log stability constant for Zn binding to human apotransferrin at pH 7.4 was estimated to be approximately 5.9 which is substantially lower than previous reports of 7.0 for the corresponding value for Zn binding to albumin (Giroux & Henkin, 1972). 3. When the relative abilities of human albumin and apotransferrin to compete for Zn with low-molecular-weight chelators were compared at the same relative concentrations of these proteins as are found in plasma, albumin retained substantially more Zn then transferrin. 4. It seems likely that albumin acts as the major transport protein for Zn in plasma of most species, Zn also being present firmly bound to alpha 2-macroglobulin. 5. In porcine plasma or serum however, there were three major Zn-binding proteins, two of which were probably albumin and alpha 2-macroglobulin. The nature of the third component remains unknown but it appeared to have a molecular weight of between 1000000-14000000, it was precipitated by 2.2 M-ammonium sulphate and by 150 g polyethylene glycol/l. 6. There were no significant differences in Zn distribution in plasma of porcine blood obtained from the aorta, the posterior vena cava or from the hepatic portal vein but use of heparin as an anticoagulant altered the normal pattern of distribution of Zn in plasma.