Abstract: We have examined retinoblastoma (RB) and P⁵³protein by immunohistochemistry inseventy-seven formalin fixed, paraffin wax embedded thyroid lessions.Thyroid lessionsstudied included 10 non-neoplastic thyroid tissues, 14 thyroid adenomas, 24 papillarycarcinomas, 19 follicular carcinomas, 9 undifferentiated carcinomas and 1 medullary car-cinoma.The RB protein is not loss in any cases, indicating that inactivatlon of the RBgene is unlikely to play a important role in the pathogenesis of thyroid tumours.Six of 9(66.7%) undifferentiated carcinonlas, 3 of 19(15.8%) follicular carcinomas, 2 of 24 (8.3%) papillary carcinomas show P⁵³protein nuclear staining.In all of P⁵³positive cases, 4of 6 undifferentiated carclnomas showed more positive cells than 5 well-differentiatedcarcinomas in which less than 10 percent of cells had increased P⁵³protein levels.Our results suggest that P⁵³protein abnormalities play a crucial role in the progression of well-differentiated to undifferentiated thyroid carcinomas.