Detection of peritubular capillary (PTC) C4d deposition in tissue sections of renal allograft biopsies became an important aid in the diagnosis of antibody-mediated rejection. Pathologists in many major transplant centers now routinely stain renal allograft biopsies for C4d. Currently, there are 3 commercially available antibodies. Two of these antibodies are monoclonal and are usually used with either a 3- or a 2-step indirect immunofluorescence (IF) methodology on frozen sections. A polyclonal antibody is used on formalin-fixed, paraffin-embedded tissue section with an immunoperoxidase detection system. The goal of our study was to compare these antibodies and methodologies in our renal allograft biopsy material. Twenty renal allograft biopsies with diffuse or focal PTC C4d staining, using immunofluorescence methods on frozen sections, were selected for this study. These biopsies were tested with the 3 commercially available anti-C4d antibodies (Biogenesis, Brentwood, Calif, cat no. 222-8004; Quidel Corporation, Santa Clara, Calif, cat no. A213; and ALPCO Diagnostic, Windham, NH, cat no. 004-BI-RC4D). Both monoclonal antibodies (Biogenesis and Quidel) were tested with a 3- and a 2-step indirect IF method on frozen sections. The polyclonal antibody (ALPCO) was applied to formalin-fixed paraffin sections using immunoperoxidase methodology. In selected cases, the polyclonal antibody was tested on frozen sections with a 3-step indirect IF method. To exclude possible false-negative staining with the IF method, we selected 10 additional biopsies that showed PTC margination of inflammatory cells, but were C4d-negative or only focally positive, and tested them with the ALPCO antibody on paraffin sections. We have found that all methodologies and antibodies tested provided adequate results with only minor differences between them. Perhaps the most sensitive method is the 3-step indirect IF on frozen sections using one of the monoclonal antibodies. We prefer the 2-step indirect IF method with the Quidel monoclonal antibody because of its simplicity, quick turnaround time, and relatively low cost. The advantages and disadvantages of the individual methodologies are discussed.