Natural heteroagglutinins for human red cells present in the serum of normal, mature rats, showing a “negative phase” in antibody titer following tumor implantation, have been characterized.

  1. The natural hemagglutinin peculiar to PA strain serum, reacting with human blood group AB and A cells, was found to be serologically related to the blood group A specific substance.

  2. None of the natural heteroagglutinins reacting with blood group B and O cells was serologically related to either the blood group B or the O (H) specific substances.

  3. The antibody causing agglutination in low titer of group B and O cells was shown to be reacting with a substance common to cells of all the four blood groups. This natural heteroagglutinin appears to be present in the serum of all three rat strains studied.

  4. Trypsinization of cells of all four blood groups appears to “uncover” an antigen common to all the cells.

  5. The ability of normal rat serum to agglutinate the normal human group B and O erythrocytes would seem to depend upon the presence of an antigen on the cell surface.

  6. There is no indication that the natural hemagglutinins characterized in this study are heterophile or anti-species antibodies.

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This investigation was supported in part by a research grant (C-1646 C3) to Dr. Paul M. Aptekman from the National Cancer Institute of the National Institutes of Health, Public Health Service.

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