In this study a double immunohistochemical staining procedure is described for the simultaneous visualization of antigen expressing cells and replicating cells. Cell surface antigen expression was marked with a monoclonal antibody against I a (His 19) or a monoclonal antibody against a membrane component of the cells of the monocyte-macrophage lineage (ED2). Replicating cells were detected by the incorporation of 5-bromodeoxyuridine. The method was applied sequentially. On frozen sections two peroxidase labeled reagents were used with two different substrates yielding a red and a dark-blue black reaction product. On plastic-embedded sections a peroxidase and an alkaline phosphatase labeled reagent were applied resulting in a brown and a blue reaction product.