Skin calcium binding protein (SCaBP) has been found in the epidermal basal layer but not in the suprabasal layers, suggesting that its presence is correlated with the position of the cell and/or the cells' low degree of differentiation and high proliferation capacity. Rats were exposed to ultraviolet B (UVB) irradiation (280-320 nm, 0.1 J/cm2) to dissociate these two main characteristics of basal layer cells. Skin biopsy specimens were taken 24 and 48 h after UVB exposure. The [3H]thymidine labeling index, SCaBP localization (indirect immunofluorescence), and SCaBP levels monitored by radioimmunoassay were investigated. The [3H]thymidine labeling index was significantly increased after UVB treatment, and the [3H]thymidine-labeled cells were present not only in the basal layer but also in the suprabasal epidermal layers. Anti-SCaBP immunofluorescence was observed in both suprabasal and basal layers (rather than exclusively in the basal layer, as in control conditions). The radioimmunoassay indicated an overall increase in skin SCaBP content. No evidence was obtained that this increase was due to humoral factors, as no changes in SCaBP concentration on cell proliferation were observed in nonirradiated epidermal areas. Topical application of a single dose of vitamin D3, equivalent to the amount synthesized by UVB exposure, was also without effect. Thus the presence of this marker is correlated with the low degree of cell maturity and the cells' ability to proliferate rather than their basal position.