1. Two preparations of bovine cytochrome c, one of 0.34 per cent, the other of 0.35 per cent iron content, were shown to be antigenic in rabbits.

  2. By the technique of “antigen absorption” it was shown that this antigenicity was not due to the presence of the colored moiety of cytochrome c but to the colorless fraction present in the cytochrome c preparation of 0.34 per cent iron content.

  3. This colorless fraction contains at least two and possibly three or more separate antigens.

  4. The bearing of these findings on the question of the purity of samples of cytochrome c containing 0.34 per cent iron is indicated.

  5. A porcine cytochrome c preparation of 0.38 per cent iron content was shown to be antigenic, whereas no evidence for the antigenicity of a sample of equine cytochrome c containing 0.43 per cent iron could be obtained.

  6. A technique of “antigen absorption” is described and its possible applications indicated.

  7. Oudin's new technique for the qualitative analysis of antigen-antibody systems was employed in demonstrating the antigenic heterogeneity of cytochrome c preparations.

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