A gel electrophoresis DNA-binding assay was used to detect proteins from herpes simplex virus type 1-infected and uninfected cells that specifically bind the upstream region of immediate-early (IE) gene 3. The assay is based on the altered electrophoretic mobility of DNA-protein complexes relative to that of free DNA in native gels. A series of end-labeled overlapping DNA fragments spanning a region from -272 to +27 (relative to the 5' terminus of the IE gene 3 mRNA) were used as probes. Two complexes were identified (referred to as A and B) which were driven by different protein factors. Formation of the A complex required infected-cell proteins extracted at any time from 2 to 16 h postinfection; a 0.5 to 1 M NaCl extract of infected cells, and a DNA probe that contained the sequences from -4 to +27 (relative to the 5' terminus of IE gene 3 mRNA). The protein that drove the formation of the A complex is not related to transcription factors TFIIIA or Sp1 or their cognate binding domains since neither the 5S RNA gene nor the GC box of simian virus 40 could compete for proteins that induced formation of the A complex. Through the use of monoclonal antibodies, the complex was shown to contain the IE gene 3 product, ICP4. A more detailed localization of the DNA-binding site in vitro by using chemical footprinting revealed that binding occurs over the sequence from -10 to +3 relative to the mRNA terminus. The binding of ICP4 to its own transcription start site may explain the repression of IE gene transcription which attends the onset of early (beta) gene expression and suggests an autoregulatory mechanism for gene control in herpes simplex virus type 1. The B complex was readily detected in uninfected cells (of a number of different cell lines), as well as in infected cells, with a probe containing the IE consensus sequence TAATGARATTC (where R is a purine) and two nested copies of the Sp1 binding motif GGGCGG; however, complexes were also detected with probes that lack the IE consensus sequence but contain Sp1 sites. These data suggest that the B complex contains the promoter-specific factor Sp1, and competition experiments with the clustered Sp1 binding domains from simian virus 40 confirmed this idea.