Immunohistochemical analysis was carried out on 144 formalin fixed paraffin embedded cutaneous melanomas to ascertain the value of 3 different immune markers. The sensitivity and staining patterns regarding intensity and distribution, as well as correlation to pigment content, cell type, surface ulceration and host response was noted. The stains used were monoclonal HMB-45 (Dako product No M634), NKI/C3 antibodies (Biogenex product No MU077) and polyclonal rabbit anti S-100A protein (Dako product No L1845). Of the lesions tested, 63 were malignant melanoma with an adjacent component of superficial spreading type, 61 were malignant melanoma with no adjacent component, 2 were malignant melanoma with adjacent lentigo maligna, 8 were in situ with 7 superficial spreading melanoma and one lentigo maligna (HMF) and 10 were metastatic melanoma. All 144 lesions stained for S-100 (100% sensitivity). One hundred and thirty-seven stained for NKI/C3 (95%); 132 stained for HMB-45 (92% sensitivity). S-100 was the most sensitive marker and stained tumor cells diffusely. With HMB-45 and NKI/C3, though marginally less sensitive, staining was stronger and patchy. In addition, NKI/C3 showed a tendency for peripheral (membrane) staining. HMB-45 staining was directly proportional to the pigment content, with stronger staining of radial growth phase melanoma and negative staining of those lesions where pigment content was minimal or absent. Also, with HMB-45 a decrease in staining intensity with depth or vertical growth phase was observed. There was no relationship to cell type with HMB-45 but with NKI/C3, 5 out of the 7 that failed to stain showed spindle cell differentiation.(ABSTRACT TRUNCATED AT 250 WORDS)