S100 proteins are calcium-activated signaling proteins that interact with target proteins to modulate biological processes. Our present studies compare the level of expression, and cellular localization of S100A7, S100A8, S100A9, S100A10, and S100A11 in normal and psoriatic epidermis. S100A7 and S100A11 are present in the basal and spinous layers in normal epidermis. These proteins appear in the nucleus and cytoplasm in basal cells but are associated with the plasma membrane in spinous cells. S100A10 is present in basal and spinous cells, in the cytoplasm, and is associated with the plasma membrane. S100A8 and S100A9 are absent or are expressed at minimal levels in normal epidermis. In involved psoriatic tissue, S100A10 and S100A11 levels remain unchanged, whereas, S100A7, S100A8, and S100A9 are markedly overexpressed. The pattern of expression and subcellular localization of S100A7 is similar in normal and psoriatic tissue. S100A8 and S100A9 are strongly expressed in the basal and spinous layers in psoriasis-involved tissue. In addition, we demonstrate that S100A7, S100A10, and S100A11 are incorporated into detergent and reducing agent-resistant multimers, suggesting that they are in vivo trans-glutaminase substrates. S100A8 and S100A9 did not form these larger complexes. These results indicate that S100 proteins localize to the plasma membrane in differentiated keratinocytes, suggesting a role in regulating calcium-dependent, membrane-associated events. These studies also indicate, as reported previously, that S100A7, S100A8, and S100A9 expression is markedly altered in psoriasis, suggesting a role for these proteins in disease pathogenesis.