The transcriptional promoter activity of the proviral long terminal repeat (LTR) of a retrovirus produced in a human lymphoblastoid cell line was studied in a variety of cell types, in comparison with that of the simian virus 40 (SV40) early promoter and Rous sarcoma virus (RSV) LTR. Recombinant plasmids containing these promoter sequences linked to the chloramphenicol acetyltransferase (CAT) gene were constructed and tested in a transient assay system to measure relative rates of transcriptional activites in different cell types, including human lymphoblastoid cell lines. The transcriptional activity of this proviral LTR was much higher than that of the SV40 early promoter and RSV LTR in various cell types, especially in human lymphoblastoid cell lines. This result is consistent with the fact that this retrovirus is a lymphotropic virus. This proviral LTR could be useful for the analyses of the expression and biological activity of various genes in human lymphoblastoid cell lines.