Cell reactivity (the peak percentage release of histamine) to ragweed antigen E and timothy pollen extract was studied in the leukocytes of children chosen for clinical ragweed sensitivity and treated with that antigen. At the beginning of therapy all children had responded to antigen E with >50% histamine release. At the time studied the cells of about one third released <50% to antigen E and their cell reactivity to timothy pollen (with which they had not been treated) correlated significantly (p <0.001). While there was a significant correlation (p <0.001 ) between the sensitivity (concentration of antigen required for 30% histamine release) and reactivity of the cells to each antigen, there was no correlation between the sensitivity of the cells to the two antigens. It is suggested that immunotherapy to one antigen decreased cell reactivity to both that antigen and to a non-cross-reacting antigen. Since decreased cell reactivity after immunotherapy has been shown by some investigators to be correlated with a positive clinical result, these studies raise a question as to the antigeníc specificity of immunotherapy.