Abstract 50Co++ added to human or rabbit plasmain vitro combines selectively with the albumin fraction. At pH 7.1, the Scatchard plot yields a curve that may be resolved for two classes of sites: one, wheren1 = 2 andK1 = 6.5 × 103, the other wheren2 = 23 andK2 = 1.6 × 102, Mn++ cannot compete for these sites. The affinity for the cobaltous ion displays a steep increment between pH 6.7 and 8.6. If injected intravenously 50% of the cobalt tracer is cleared from the plasma in 25 minutes. An additional complex of higher molecular weight than that of albumin is formedin vivo.