A heterogeneous group of 159 tumours was studied for the presence of S-100 protein by the immunoperoxidase technique in order to determine whether this marker may be of value in facilitating immunocytochemical diagnosis. Among cases of melanocytic and pigmented lesions, S-100 was widely distributed and demonstrated the strongest degrees of reactivity. S-100 protein was identified in virtually all nerve sheath tumours such as schwannomas, neurofibromas, myxoid sheath nerve tumour and also in some tumours of controversial histogenesis such as granular cell tumours. The great majority of carcinomas did not express S-100, with only two cases of breast carcinoma displaying focal S-100 staining. In a miscellaneous group of tumours S-100 was demonstrated in chordomas, myoepitheliomas and Wilms' tumour with Schwann cell differentiation. Despite its presence in a wide array of cell types, S-100 protein continues to be an extremely useful marker especially for soft tissue and peripheral nervous system tumours.UNIV ESTADUAL PAULISTA JULIO MESQUITA FILHO, BOTUCATU MED SCH, FAC MED,DEPT PATOL, BR-18610 BOTUCATU, SP,BRAZILUNIV ESTADUAL PAULISTA JULIO MESQUITA FILHO, BOTUCATU MED SCH, FAC MED, DEPT PATOL, BR-18610 BOTUCATU,SP,BRAZIL