We investigated permissiveness of the malignantly transformed trophoblast (choriocarcinoma) cell lines JAR, BeWo and JEG-3 to the human T cell lymphotropic virus type I (HTLV-I). After co-culture with the productively infected cell line MT-2 the choriocarcinoma cell lines were analysed for infection over a period of three months. The presence of HTLV-I viral DNA was examined by PCR using primers targeting the gag, pol, env and pX specific sequences. All amplified segments were found consistently in the cell cultures throughout the period of study. Further analysis that aimed to characterize the size variation of the integrated proviral DNA by Southern blotting revealed the presence of integrated proviral sequences which consisted, for the most part, of incomplete genomes. The presence of the full-length HTLV-I genome, however, was unequivocally confirmed in clonally expanded cell cultures derived from the originally infected parental cells. In order to analyse virus expression at the transcriptional level, we used reverse transcriptase (RT)-mediated PCR that was targeted at intraexon regions (gag, pol, env and pX), and the splicing sites of the env and pX-tax/rex mRNAs. When compared with MT-2 cells, substantially lower levels of all transcripts were found in all the cell lines analysed. We were unsuccessful in attempts to detect viral protein expression using polyvalent or Tax- and Gag-specific monoclonal antibodies by Western blot analysis or immunoprecipitation, and we could not detect any RT activity released into the supernatant of the infected cells either. Collectively, these data suggest that the trophoblastic cells may become persistently but essentially non-productively infected with HTLV-I.