The resemblance between retrotransposons and retroviruses suggests an evolutionary relationship and indicates that they may share common transcription factors. We have analyzed the behaviour of the Drosophila 1731 retrotransposon promoter in the human monocytic U937 cell line. We show that the long terminal repeat (LTR) of 1731 promotes CAT (chloramphenicol acetyl transferase) activity in these cells, in which it is enhanced by phorbol esters. Using gel mobility assays, we detected a human nuclear protein that binds in the U3 region of the LTR in a sequence-specific manner. Its precise target was determined by a DNase I footprinting experiment.