The mechanism of cellular transformation by the human T-cell leukemia viruses (HTLVs) is thought to involve a novel retrovirus gene known as chi. The chi gene is essential for HTLV replication and acts by enhancing transcription from the viral long terminal repeat. By using the HTLV type I and II chi gene-coding regions inserted into a highly efficient expression vector, we directly compared the efficiencies of the two chi proteins to trans activate the HTLV type I and II long terminal repeats. We demonstrate that the two chi proteins have different patterns of trans activation. The patterns were highly reproducible in all mammalian cells tested. A different pattern of activation was observed in avian cells. These results suggest that the mechanism of trans activation involves specific cellular factors that are highly conserved throughout mammalian species but different in avian cells. Understanding the mechanism of trans activation by the chi gene product may provide insights into mechanisms of cellular transformation by HTLV.