The specific DNA-binding Gal4 amino-terminal portion (amino acids 1 to 147) was fused to protein segments of BLV transactivator p34tax and tested for its capacity to activate CAT-gene expression in mammalian cells. The p34tax peptide segment 157 to 197 encompasses an activating region. The segment is approximately located in the middle of p34tax and is globally neutral (net charge zero). The tax (157-197) domain contains 24% of leucine residues, possibly involved in heterologous protein interactions.