Forty-seven samples of paraffin-embedded formalin-fixed (and 25 related frozen) sections of 27 primary pleomorphic adenomas, 15 recurrent pleomorphic adenomas and 5 carcinomas in pleomorphic adenomas were studied to analyse their immunohistologic patterns with respect to the ratio of the expression of 'normally' and 'aberrantly' differentiated cell types. In primary pleomorphic adenoma PTHrP-positive cells are seen in the inner layer of tubulo-ductal structures, in part of the cells in the mucoid, chondroid, or myxochondroid matrix, and in the squamous metaplastic areas. Bcl-2-positive cells are found in the outer layer of tubulo-ductal structures, in part of the cells in the mucoid, chondroid, or myxochondroid matrix, and around the squamous metaplastic areas. In one case of primary pleomorphic adenoma, which recurred later, the positivity for Bcl-2 is more intense and seen in the periphery of this tumour with a predominantly myxoid pattern. In recurrent pleomorphic adenomas, which also mostly showed a predominantly myxoid pattern, the positivity for Bcl-2 showed a pattern similar to the primary-to-recur tumour. PTHrP-positive cells are found less frequently than Bcl-2-positive cells. In carcinoma in pleomorphic adenoma, the benign part shows the features of primary pleomorphic adenoma with its Bcl-2 and PTHrP-positivity patterns. The malignant part strongly shows Bcl-2-positive cells in the periphery of the tumour. We conclude that the maintained presence of Bcl-2 and PTHrP-positive cells in the tumours we studied shows the variable capacity of tumour cells to differentiate.