The testis is an immunologically privileged site. Maintenance of an immunosuppressive environment is crucial for sperm production and any compromise can lead to male infertility. Immune cell populations have been characterised in sexually-mature rodent testes [1-3], but knowledge of these cells during the first spermatogenic wave is limited. Because immune cells and their products, including cytokines, influence organ development, we hypothesized that testis development would be associated with significant changes in immune cell subpopulations. In order to address this, we have characterised two strains of reporter mice, as previously employed to analyse immune cells in the mouse epididymis . Macrophages were visualised using the CX3CR1-GFP knockout mouse model  and dendritic cells with a CD11c-YFP transgene . Testes were fixed in 4% paraformaldehyde/PBS immediately after collection for 5 hours at RT, passed through a sucrose gradient, then frozen in Tissue Tek OCT. Sections of 25μm were stained with DAPI and imaged using a Leica SP8 confocal microscope. Substantial numbers of macrophages (CX3CR1-GFP+ cells) were present within the interstitium at 7 and 14 days post-partum (dpp). Smaller numbers of CD11c-YFP+ cells were also present in the interstitial space. An additional YFP signal was visible throughout the Sertoli cell cytoplasm at 7 dpp, although not at 21 dpp. This result suggests that the CD11c promoter, normally active in dendritic cells, is transiently active in proliferating Sertoli cells, which may reflect some aspect of their role in promoting immunoregulation in the juvenile testis. These mouse models will enable the use of section co-staining with markers for distinct immune cell populations as well as flow cytometry to characterize and quantify the immune cells and their subtypes during key stages of mouse testicular development.