Dialysis experiments were performed with an acetylcellulose membrane between two identical sample solutions; a trace amount of radiolabelled palmitate was added on one side and the rate of dialytic equilibration of the label was measured. By comparison with rates measured in standard experiments, using pure albumin solutions, we obtained the reserve albumin concentration for binding of palmitate, previously defined as the concentration of a pure standard albumin which binds the labelled ligand as tightly as it is bound in the sample. Two techniques were developed, one for 1-ml sample volumes, another for 25 microliter. Reserve albumin for binding of palmitate, measured in pure albumin solutions, decreased equally on addition of palmitate and stearate, slightly less with oleate and still less with linoleate, indicating that palmitate and stearate are bound competitively while interaction with binding of the unsaturated acids is less pronounced. Chloride ions compete with binding of palmitate. Reserve albumin concentration in serum samples from 33 male adults was 420 +/- 59 microM (mean +/- SD), and in 33 females, 351 +/- 50 microM. Umbilical cord sera from ten newborn infants averaged 172 +/- 56 microM. The reserve albumin concentrations in the sera of newborn infants are low compared with the normal albumin concentration, which is about 600 microM. This appears to indicate the presence of an albumin species with lower affinity for palmitate.