Neoplastic and non-neoplastic oxyphil cells in thyroid disorders were investigated for enzyme and synthetic activity, ultrastructural features, and immune complex deposition. A reduced but definite content of thyroglobuliin and similar enzyme profiles and ultrastructural features in all types of oxyphill change were found. Immune complex deposition was seen in Graves' disease and Hashimoto's thyroiditis but not in oxyphil neoplasms or colloid goitre with focal oxyphil change. The findings suggest that oxyphil cells do represent a uniform change. Immunological damage with immune complex deposition might be the cause of oxyphil change in only some conditions.