The blood of a patient with infectious mononucleosis was investigated when a discrepancy was noted between cell and serum grouping. The red blood cells of the patient were reactive with a commercial anti‐B. Her serum strongly agglutinated group A and group B cells. Anti‐B from four other commercial sources and from ten normal group A donors did not agglutinate the patient's cells.
The coloring material in the commercial anti‐B with which the patient's cells reacted is yellow #5 tartrazine. The patient's serum agglutinated normal group O cells on the addition of this dye. No other example of this factor has been found in routine blood grouping or in testing the serum from 30 further patients with infectious mononucleosis.