PURPOSE. The ability to mount antigen-specific immune reactions in the retina demonstrates local recognition of retinal antigens. However, properties of antigen-presenting cells (APCs) of the retina are uncertain. The current study was undertaken to look for evidence of CD45+ cells with APC potential in the retina and to examine their in situ and in vitro responses to IFN-γ and anti-CD40, two stimuli known to upregulate activities associated with antigen presentation. METHODS. Mice were pretreated with systemic or intracameral (IC) inoculations of IFN-γ or anti-CD40. Retinas were harvested, enzymatically dissociated, positively' selected with anti-CD45, and analyzed by flow cytometry with antibodies known to identify APCs. RESULTS. The most common CD45+ retinal cells were CD11b+, F4/80+, CD8α+, CD80+, and major histocompatibility complex (MHC) class IIlo, a phenotype characteristic of central nervous system (CNS) microglia (MG). There was also a small population of DEC-205+ cells and a smaller number of CD11c+ cells, both markers of dendritic cells (DCs). IC inoculation of IFN-γ led to an increase in the number of CD45+ cells and a modest upregulation of MHC class II on CD11b+ cells. IC inoculation of anti-CD40 also increased the total number of CD45+ cells and the number of CD11b+ cells, but increases in CD80 and MHC class II expression on CD11b+ cells were insignificant. After anti-CD40 treatment, CD45hi11c+ cells increased in number and altered their expression of CD11b. CONCLUSIONS. Retinal MG were readily identified as the most numerous population. A small population of cells with perivascular cell (PVC)-like properties was found. They were CD45hi11c+, some had elevated MHC class II, and they were affected by anti-CD40 treatment in vivo. No conventional DCs were found, although there was a distinct DEC-205+ population. Overall, the effects of IFN-γ and anti-CD40 treatment were attenuated in the retina in vivo and also on CD45+ cells in culture, compared with the control.