Background: Peroxidase content has been recently evaluated in normal thyroid and in different thyroid disorders by biochemical, histochemical, ultrastructural and immunocytochemical methods Nevertheless immunocytochemical detection of thyroid peroxidase in thyroid samples conventionally processed for histology hes never been done using a commercially available antibody, neither its correlation with the biochemical activity on adjacent samples. Methods: In this study we have analyzed normal thyroid tissue (3 patients), follicular adenoma (2 patients) and multinodular goiter (2 patients) conventionally processed for histology and stiined by immunocytochemistry (Avidin Biotin System) using a polyclonal (rabbit) antibody for horseradish peroxidase (Serotec). Biochemical assay was performed on adjacent samples according to Hosoya method. Results: Normal thyroid showed peroxidase immunoresctivity in the majority of follicular celis; neoplastic cells of adenomas were variably stained. Biochemical assay showed positive correlation with ICC ranging from 20.4 mg/mg/prot a in multinodular goiter to 42.12 in normal thyroid, up to 122 of follicular adenoma. Conclusions: Peroxidase content in the thyroid gland may be of clinical interest in several thyroid disesses, and in this study we have demonstrated that thyroid peroxidase can be detected by ICC in routinely processed thyroid samples using a commercially available antibody.