The authors determined the agglutination percentages with anti-B, eel and Ulex europeus anti-H reagents for the kinetic study of the in vitro conversion O red blood cells in B. The agglutination with the anti-B increases in proportion as the agglutination with eel anti-H decreases; the agglutination with Ulex anti-H remains constant. They converted (time=18 h) O red blood cells with a more or less high substance H content (adults in good health, africans and europeans, patients, newborn and one "Bombay" phenotype). They showed that there is a good correlation between the agglutination percentages with eel anti-H or Ulex anti-H before conversion and the agglutination percentages with anti-B after conversion. These "O converted" look like the B phenotype defined by a check sample of B subjects.