SOME applications of the polarographic technique have been made to the investigation of metal binding by proteins, especially serum albumin1-4. Tanford has shown that the polarographic behavior of thallous ion is unaffected by the presence of bovine serum albumin over a considerable pH range. As a preliminary study in a programme of work on the combination of metals with gelatin, I have examined the effect of gelatin on the polarography of thallous ion. The gelatin used was a high-grade lime processed hide gelatin, of molecular weight 79,000 (light scattering). It was carefully de-ionized to give an ash content of < 0.02 per cent. The moisture content was 12.7 per cent; protein concentrations refer to the w/v concentration of dry, ash-free protein. The polarograph used was a linear sweep cathode-ray instrument (Southern Analytical Ltd., type K1000). Polarograms were measured at 25.0° C and all potential values are corrected to the standard calomel electrode scale. Solutions were de-oxygenated with a slow stream of nitrogen. With the design of cell supplied with the polarograph, it was possible to transfer about 2 ml. of the solution under test and pass the gas at a sufficiently slow rate for the froth not to be excessive. De-oxygenation was normally satisfactorily complete in about 20 min.