The mechanism of trans-acting regulation of transcription from the long terminal repeat (LTR) of human immunodeficiency virus (HIV) has been investigated. The roles of the cis-acting elements within HIV LTR, as well as the trans-acting factors present in HIV-infected cells, have been evaluated by an in vitro transcription system. Our observations indicate that both the sequence downstream from the CAP site of HIV LTR (located at nucleotide positions +1 to +56; called the TAR element) and the GC boxes (-77 to -45) are required for full transcriptional stimulation and that both the virus-encoded tat protein and one or more cellular factors might be involved. These results demonstrate the presence of a combinatorial regulation of HIV transcription by multiple factors, which may confer the provirus with greater flexibility in regulated viral gene expression.