Leucocytes from normal non-allergic donors were passively sensitized with sixty-five atopic sera that contained IgE directed against house dust mite (HDM) or guinea-pig dander (GPD). In no serum was there any IgE with an abnormal basophil anaphylactic-sensitizing activity. In sera from patients hypersensitive to GPD, both GPD-specific IgE and IgG4 were measured. A significant correlation was found between the GPD-IgE RAST and the basophil-sensitizing capacity of GPD-positive sera, but no correlation was found between this sensitizing capacity and the GPD-IgG4 RAST. Leucocytes were passively sensitized with serum mixtures that contained the same amounts of total and HDM-specific IgE; these mixtures differed as to sensitizing activity. It is unclear whether this variation is due to differences in IgE or to influences of other factors in serum.