The parvovirus H-1 P38 promoter contains a trans-activation responsive element (tar). It was previously shown that the parvovirus H-1 nonstructural protein NS1 positively regulates the expression of the P38 promoter for the viral capsid protein gene via the tar (Rhode and Richard, 1987, J. Virol. 61, 2807-2515). To characterize the mechanism of trans-activation by the tar, we used gel shift assays to demonstrate that there exist proteins in virus-infected cellular extracts which have higher binding activity than that found in mock-infected extracts. These observations in vitro are consistent with the expression by P38 constructs with the wild-type promoter linked to a reporter gene, chloramphenicol acetyl transferase (cat), in vivo. We also provide evidence that the protein(s)-tar complex has a molecular mass of approximately 75 kDa in an SDS-polyacrylamide gel, which is less than NS1, and this complex cannot be precipitated by NS1 antibody, which suggests that NS1 mediates the trans-activation by inducing an alteration in the binding activity of some cellular protein(s) in an indirect manner. These data support our previous hypothesis for the activation of the P38 promoter, in which the trans-activator(s) interacts with the tar effectively in the presence of NS1, leading to the formation of the transcription initiation complex by protein-protein associations (Gu, Chen, and Rhode, 1992, Virology 187, 10-17).