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 degrees 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.