Binding of the anti-inflammatory drug niflumic acid to serum albumin was measured by equilibrium dialysis and the dissociation constants were determined. The maximal binding capacity was 36 mol niflumic acid per mol albumin. Most of the binding sites were of low affinity, only six having dissociation constants below 1 mM. At the plasma concentrations most frequently used in experimental work, the high affinity sites account for more than 99% of the albumin-bound niflumic acid.