The cell surface expression of low-affinity Fc epsilon receptor (Fc epsilon RII), which is identical to CD23, and the serum levels of IgE binding factors (IgE BFs), a soluble form of Fc epsilon RII, and IgE were investigated in HTLV-I-infected individuals, and the results were compared with those for HTLV-I-negative controls. The occurrence of Fc epsilon RII+ mononuclear cells did not differ among the blood samples from the HTLV-I-infected groups; however, the levels of serum IgE BFs were significantly decreased (P less than 0.001) in patients with adult T cell leukaemia (ATL), compared with HTLV-I-negative controls. The serum IgE levels were also significantly decreased in these groups, including cases of ATL (P less than 0.001) and HTLV-I-associated myelopathy (P less than 0.05), and HTLV-I carriers (P less than 0.05), as compared with HTLV-I-negative controls. Since cases of strongyloidiasis or filariasis are frequently found among HTLV-I-infected individuals in Japan, these results may explain in part their defective defence mechanisms for parasites. The possible involvement of IgE BFs in the development of immune deficiency state in HTLV-I infection is also discussed.