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Notes: Several in-vitro experiments suggest that the low affinity receptor for IgE (FcɛR II) and its soluble fragment (IgE-binding factor, IgE-BF) are multi-functional molecules and more particularly that they are capable of regulating the synthesis of human IgE. I nan attempt to examine the in-vivo significance of these in-vitro observations, the serum level of IgE-BF was measured in individuals with allergic or parasitic diseases, both associated with an increased production of IgE. IgE-BF was measured by a radioimmunoassay employing two mAbs against FcɛRII (mAbER). We first compared 257 allergic subjects to 172 non-allergic controls matched for age and sex. Statistical analysis of the data, after logarithmic transformation of IgE-BF and IgF. values, revealed that despite a great overlap, the allergic subjects had significantly higher levels of IgE-BF. The correlation between IgE and IgE-BF was very weak but significant. Allergic or non-allergic children had significantly higher IgE-BF levels than the corresponding groups of adults; moreover, the in verse correlation between age and IgE-BF levels was significant only in the children and not in the adults. The IgE-BF levels were not influenced by gender, by hyposensitization therapy or by treatment with local steroids. Subjects receiving systemic steroids had lower IgE-BF levels than untreated subjects. To assess the effect of seasonal exposure to allergen, IgE-BF levels were monitored for 1 year in six rye grass-sensitive hay fever subjects known to display a seasonal increase of both total serum IgE and of specific anti-Lol p I and Lol p II IgE antibodies. The levels oflgE-BF remained stable in the hay fever subjects as well as i n the non-allergic controls. Twenty subjects with filariasis and high serum IgE concentrations also displayed increased levels of serum IgE-BF when compared to control individuals. The serum IgE-BF detected by the RIA was not bound to IgE as shown by two independent observations: (1) they were not adsorbed on an anti-IgE immunoadsorbent column and no serum IgE was retained on an immunoadsorbent column coupled to mAb against IgE-BF; and (2) the serum molecules binding to mAbER-coated solid phase did not react with radiolabelled monoclonal or polyclonal anti-IgE, and reciprocally the serum molecules binding to anti-IgE-coated solid phase did not react with labelled mAbER. Taken collectively, the present data are compatible with, but do not add further support to, the hypothesis that IgE-BF may regulate the synthesis of human IgE.