Immunohistochemistry (IHC) assays provide valuable insights into protein expression patterns, the reliable interpretation of which requires well-characterized positive and negative control samples. Because appropriate tissue or cell line controls are not always available, a simple method to create synthetic IHC controls may be beneficial. Such a method is described here. It is adaptable to various antigen types, including proteins, peptides, or oligonucleotides, in a wide range of concentrations. This protocol explains the steps necessary to create synthetic antigen controls, using as an example a peptide from the human erythroblastic oncogene B2 (ERBB2/HER2) intracellular domain (ICD) recognized by a variety of diagnostically relevant antibodies. Serial dilutions of the HER2 ICD peptide in bovine serum albumin (BSA) solution are mixed with formaldehyde and heated for 10 min at 85 °C to solidify and cross-link the peptide/BSA mixture. The resulting gel can be processed, sectioned, and stained like a tissue, yielding a series of samples of known antigen concentrations spanning a wide range of staining intensities. This simple protocol is consistent with routine histology lab procedures. The method requires only that the user have a sufficient quantity of the desired antigen. Recombinant proteins, protein domains, or linear peptides that encode relevant epitopes may be synthesized locally or commercially. Laboratories generating in-house antibodies can reserve aliquots of the immunizing antigen as the synthetic control target. The opportunity to create well-defined positive controls across a wide range of concentrations allows users to assess intra- and inter-laboratory assay performance, gain insight into the dynamic range and linearity of their assays, and optimize assay conditions for their particular experimental goals.