Abstract Pooled plasma samples from laying hens and from cockerels were incubated in vitro with 45Ca and aliuots were periodically withdrawn for ultrafiltration. The specific activity of calcium in the ultrafiltrates from cockerel plasma did not significantly change during the 2.5 h of incubation. With laying hen plasma, there was a gradual decrease in the specific activity of the ultrafiltrates, suggesting the presence of a slowly exchanging fraction in the protein-bound calcium. The size of this fraction was estimated, by kinetic analysis, to be 4.3 mg per 100 ml out of a total of 29.0 mg per 100 ml, and its exchange rate was estimated as 0.11 mg/min per 100 ml. The significance of the presence of this fraction in the plasma in studies of tracer kinetics is discussed.