In vitro IgE and antimite IgE antibody (IgE Ab) synthesis was investigated. Peripheral blood lymphocytes (PBL) from atopic patients depleted of E-rosetting T cells developed IgE and IgE Ab in the absence of pokeweed mitogen (PWM), and it was not significantly affected by addition of T cells of of PWM. 1,000 R irradiation decreased more than 50% of the IgE and IgE Ab-producing capacity in 2 of 8 cases, but the remainder were not significantly affected. Increase of the gamma-ray dose from 1,000 to 12,000 R did not result in more suppression. Radio-resistant as well as radio-sensitive subpopulations may exist in the PBL of atopic patients. The radio-sensitive component of IgE and IgE Ab formation was suppressed by PWM, while the radio-resistant component was not affected.