In contrast to IgE, myeloma IgG4 did not block passive sensitization of basophil leukocytes by IgE antibody. Atopic sera, selected for a high level of mite-specific IgG4, lost their sensitizing activity after they had been heated at 56 °C. After incubation of leukocytes with myeloma IgG4 or with sera with high levels of specific IgG4, the cells did not become responsive towards anti-IgG4 antiserum. After incubation of leukocytes with fluorescein-labelled myeloma IgE, these cells released histamine with antifluorescein antiserum. In contrast, after incubation with fluorescein-labelled myeloma IgG4, a subsequent incubation with antifluorescein antiserum did not result in histamine release. Therefore, we cannot confirm the hypothesis of Vijay and Perelmutter that IgG4 can sensitize basophil leukocytes in vitro.