Peptides of the herpes simplex virus type 1 regulatory protein, ICP4, which are translated from genes containing nonsense and deletion mutations retain specific biochemical properties and activities characteristic of the intact ICP4 molecule (N. A. DeLuca and P. A. Schaffer, J. Virol. 62:732-743, 1988). Mutant viruses expressing these peptides are deficient for viral growth in the absence of complementing wild-type protein supplied in trans, indicating that the mutant peptides are not functionally complete. In the present study we have demonstrated that certain pairs of mutants expressing partial ICP4 peptides complement each other. The complementation is shown at the level of transcription and results in enhanced virus growth. Among complementing pairs of ICP4 mutants is a virus expressing a peptide deleted for codons 185 to 309 (d2) and a virus expressing only the amino-terminal 774 amino acids (n208). By using a mobility-shift assay and by taking advantage of the specific DNA-binding properties of ICP4, it was demonstrated that novel ICP4-containing DNA-protein complexes were found when extracts from cells coinfected with complementing pairs of ICP4 mutants were incubated with target DNA. The novel complexes were shown to be a function of both mutant peptides in the coinfected cell, suggesting that complementation results from the multimerization of partial ICP4 peptides.