We established an avidin-biotin method for the sensitive determination of rat IgE and found that a non-specific signal was generated depending on the reaction temperature. When the sera of rats immunized or not with ovalbumin (OVA) were fractionated in a hydroxyapatite column and OVA-specific IgE was determined by the avidin-biotin method at 4 degrees C or 37 degrees C, OVA-specific IgE peaks were detected at 37 degrees C, even with nonimmunized rats, but not at 4 degrees C.