1. The binding of lead to human blood serum, and components of serum, was studied by titration with the addition of Pb(NO 3 ) 2 solution, monitoring the free Pb 2+ concentration with a Pb 2+ electrode, and by equilibrium dialysis. 2. In fresh serum, about 4999 out of 5000 parts of added lead were bound. This suggests that the free Pb 2+ concentration is around 1/5000th of the total lead concentration in the serum of normal subjects, i.e. about 1 × 10 −12 mol/l. 3. About 60% of the binding of lead in serum is abolished by standing in air, by dialysis or by treatment with N -ethylmaleimide. This appears to be due to the presence of thiol compounds, mainly cysteine. The remaining 40% appears to be due to protein, mainly albumin.