1. A protein, aggregating at body temperature and solubilizing when cooled, was isolated from fresh human milk at neutral pH and studied for some of its physical, chemical and immunological properties. The name `galactothermin9 is proposed for this protein. 2. Isolation and purification of galactothermin involved casein removal from skim milk at pH4.64 followed by centrifugal fractionation of residual protein-containing solutions repeatedly heated and cooled between 40°C and 0°C at pH7.3. 3. The molecular weight by ultracentrifugal analysis and the minimum molecular weight by sum of amino acid residues were 11400 and 14000 respectively. The sedimentation coefficient s25,w was 1.05S and the diffusion coefficient was 7.15×10−7. Reversible aggregation is favoured by increase in protein concentration, ionic strength, temperature, time and approach to the isoionic point of 7.27 from either acidic or alkaline conditions. 4. Among the amino acid residues, proline predominates and non-polar species account for two-thirds of the total. Cysteine and cystine are absent. Analysis of galactothermin showed it to be essentially free of hexose, sialic acid, calcium and phosphate. 5. Galactothermin is antigenic in the rabbit as evidenced by the passive cutaneous anaphylaxis test. Single precipitin lines are produced in immunodiffusion tests. 6. By electrophoresis in polyacrylamide gel at pH4.0 only one sharp band is produced.
