1. Human haptoglobin type 1-1, porcine haptoglobin, and equine haptoglobin were isolated and purified. 2. These haptoglobins were similar in polyacrylamide gel electrophoresis and in subunit structure but showed microheterogeneity in isoelectric focusing. 3. Isoelectric points of human haptoglobin as determined with photopolymerized gels were found to be 4.03-4.24, of porcine haptoglobin 4.0-4.30, and of horse haptoglobin 3.80-4.15, respectively. 4. Results obtained with chemically polymerized gels were 0.08-0.3 pH units higher. 5. Examined haptoglobins differed also in the ability of complex formation with hemoglobin, in sialic acid content and in antigenic specificity.