AbstractProliferating cell nuclear antigen (PCNA), an endogenous cell replication marker, was detected by an improved immunohistochemical procedure (super sensitive biotin-streptavidin), in formalin fixed, paraffin embedded tissue. Small intestine, liver, kidney, testis, and esophagus from rats were stained for the detection and evaluation of PCNA as a suitable marker for cell proliferation. Different patterns of staining that are believed to correlate with the individual phases of the cell cycle (G1, S, G2, M, G0) were recognized. PCNA offers an alternative to established cell labeling techniques for investigating cell proliferation. (The J Histotechnol14:237, 1991)