Seminal fluid enhanced human T-cell leukemia virus type 1 (HTLV-1) infection by transactivating the HTLV-1 long terminal repeat promoter, which is chromosomally integrated in a cell-type-dependent manner. Our data may indicate a potential role for seminal fluid in the sexual transmission of HTLV-1 and imply complex features of regulation of HTLV-1 expression.