Tissue localization of immune cells is critical to the study of disease processes in mouse models of human diseases. However, immunohistochemistry (IHC) for immune cell phenotyping in mouse tissue sections presents specific technical challenges. For example, CD4 and CD8 have been difficult to detect using IHC on formalin-fixed and paraffin-embedded mouse tissue, prompting alternative methods. We investigated the use of formalin-free zinc-salt fixation (ZN) and optimized IHC protocols for detecting a panel of immune cell–related markers (CD3, CD4, CD8, Foxp3, B220, F4/80, CD68, and major histocompatibility complex [MHC] class-I, MHC class-II, and Gr-1). The IHC results for these markers were compared on mouse spleen tissue treated with neutral buffered formalin (NBF) or ZN with or ZN without antigen retrieval (AR). Whereas CD4 and CD8 were not detected in NBF-treated tissue, all markers were detected in ZN-treated tissue without AR. Thus, the use of ZN treatment for IHC staining can be a good tool for studying immunoreactive lesions in tissues.