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Staphylococcal hemagglutinating antibodies are present in both 7S and 19S fractions of serum. Newly born infants receive only 7S hemagglutinins from their mothers, and this passively acquired antibody soon disappears from the circulation. Infants under 4 months of age fail to respond to severe staphylococcal infections with hemagglutinin production. Infants over 4 months of age do make hemagglutinins, but these antibodies are mostly in the 19S class. Infants and children over 4 months of age may have high titers of hemagglutinins even without a history of staphylococcal infections. Most adults have significant staphylococcal hemagglutinin titers. The distribution of staphylococcal hemagglutinating antibodies between 7S and 195 is consistent. Under the age of 16 years, most of the actively acquired hemagglutinin is in the 19S class. After 16, 7S hemagglutinins may dominate. Thus, under the conditions of natural exposure to staphylococci, age of the individual is a significant determinant of the class of his staphylococcal hemaglutinating antibodies. Although it is clear from this and other reported studies that individuals react to different antigenic stimuli by making different classes of antibodies, the biological and clinical significance of these observations are not yet completely understood.