The purpose of this study was to evaluate the expression of the Ca(2+)-binding S100 proteins S100A1, S100A2, S100A3, S100A4, S100A6 and S100B in normal skin. These immunohistochemical staining patterns were compared with those in melanocytic lesions. Paraffin-embedded tissue of normal skin adjacent to 26 naevi, 39 primary cutaneous melanomas and 14 cutaneous melanoma metastases was incubated with polyclonal antibodies against the recombinant human S100 proteins (S100A1, S100A2, S100A3, S100A4, S100A6, S100B) using the alkaline phosphatase anti-alkaline phosphatase method. The S100A2 antibody stained the basal layer of the epidermis and hair follicles of normal skin. Four of 39 primary cutaneous melanomas were positive for S100A2, whereas none of the metastases or naevi showed any immunoreactivity. The S100A3 antibody only stained the inner root sheath cuticle of some hair follicles but no melanocytes or melanocytic lesions. Staining of S100A4 was weak and thus omitted to further analysis. S100A6 faintly labelled keratinocytes. Langerhans' cells, melanocytes and sweat glands. S100A6 immunoreaction was found in two of seven junctional naevi, five of seven compound naevi, and all dermal and blue naevi. There was an intense cytoplasmatic reaction for S100A6 in all primary cutaneous melanomas and in nine of 14 (64%) metastases, S100B was positive in melanocytes and Langerhans' cells, all primaries as well as in the metastases, S100A1 protein was not detected on any of the tissue specimens examined. Whereas S100B and S100A6 antibodies are useful markers for malignant melanoma, expression of S100A4 antibody is too low to be used for immunohistochemical staining. S100A1 and S100A3 antibodies are not expressed in melanocytic lesions and S100A2 is only found in selected tumours. The investigated S100 proteins, including S100B and S100A6, are also expressed in selected elements of normal skin. These findings are important for correct interpretation of staining patterns, when S100 antibodies are used as markers for melanoma or other tumours.