Pregnant women tested positive for antibodies to human T cell leukemia virus-I (HTLV-I), even though they had tested negative during a previous pregnancy. We investigated the correlations between production of HTLV-I antibodies (HTLV-I Ab) , expression of HTLV-I antigen (HTLV-I Ag), and proliferation of cells due to estrogen in vitro. Peripheral blood lymphocytes (PBLs) were separated from five HTLV-I sero-positive non-pregnant women and titration of HTLV-I Ab in sera undertaken. Cultured PBL were stained by indirect inrmunofluorescence for detection of HTLV-I Ag expression. It was shown that 17-β estradiol (E2) increased HTLV-I Ab production and expression in cultured PBL from sero-positive women. These results suggest that pregnancy stirnulates the production of HTLV-I Ab in women infected with low concentrations of HTLV-I, but who appear sero-negative, and that this is the reason why some pregnant women tested positive even though they had tested negative during a prevlous pregnancy. identifier:Journal of Nara Medical Association Vol.55 No.2 p.81-86 identifier:13450069 identifier:http://ginmu.naramed-u.ac.jp/dspace/handle/10564/225 identifier:Journal of Nara Medical Association, 55(2): 81-86