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A cDNA clone of the bovine immunodeficiency-like virus (BW) trans-activator gene (tat) was identified and characterized. The tat cDNA clone was generated by splicing, and on the basis of sequence analysis, the Tat protein was found to be encoded entirely by the first exon. It is 103 amino acids in size and shares sequence homology with the human immunodeficiency virus (HW) Tat. The BIV tat clone can trans activate the BIV promoter effectively, as measured by the expression of the bacterial chloramphenicol acetyltransferase gene, when transfected into bovine cells. Besides activating the BIV promoter, the BIV Tat can also trans activate the HIV promoter effectively. It is possible that BIV Tat and HIV Tat employ similar mechanisms in trans activation of the viral long terminal repeat-directed gene expression.