During the last several decades, immunohistochemical studies of tumors, along with other approaches, have suggested that the clinical and biological progression results, at least in part, from the sequential appearance within the neoplasm of cellular subpopulations whose new characteristics reflect specific somatic genetic changes. However, CNS may provide a different microenvironment for activation and proliferation than other tissues. The tissue-specific distribution of intermediate filament proteins, in particular the keratins, permits their use as marker in histopathology, but several important exceptions are recognized. In this connection, it is of interest that, according to the other reports, glial tumors may be positive for different anti-keratin antibodies. However, the gliomas did not show an immunoreaction in any of the cases when HEA-125 and Ber-EP4 were applied. The great number of multihormonal pituitary adenomas and possible change of the immunohistochemically detectable hormone status in cases of recurrent tumors have particularly re-emphasized the need for new thinking about patterns of classification. The diagnosis of malignant melanoma has been considerably facilitated recently by the introduction of immunohistological labelling with antibodies selective against melanoma antigen (HMB-45). Our results confirmed the necessity of cautious interpretation of HMB-45 immunoreactivity because a HMB-45 expression can be observed in several non-melanotic tumors.