The use of specific cytokeratin (CK) expression is a well-established tool not only in the differential diagnosis of human lung cancer but also in mechanistic investigations of various animal nonrespiratory carcinogenesis models. We previously analyzed CK expression changes in the rat respiratory tract following exposure to room-aged cigarette sidestream smoke (RASS) and in a limited number of spontaneous lung tumors. The aim of this investigation is to corroborate the specificity of CK expression patterns for certain rat lung tumors and their preneoplastic precursors. CK expression was studied immunohistochemically in serial paraffin sections from rat lung tumors and precursor lesions from an inhalation study (male and female wistar rats, 24 mo. Exposure, 6 mo. Postexposure, 6 h/d, 7 d/wk, nose-only, 3 and 10 mg/m(3) particle concentration) with RASS and diesel engine exhaust (DEE). A total of 73 lung tumors (44 bronchiolo-alveolar (BA) adenomas, 10 benign keratinizing cystic squamous cell (BKC) tumors, 2 BA carcinomas, 2 squamous-cell carcinomas, 1 nonkeratinizing squamous-cell carcinoma) and their precursor lesions as well as 14 metastases were immunostained for CK7, CK18, CK19, CK4, CK14, CK15, CK1,10/11. Expression of CK19 was seen in all epithelial tumors and precursor lesions. BA adenomas and BA carcinomas expressed CK7and CK18, as did most of the BA hyperplasia and BA metaplasia. The DEE-specific squamous metaplasia, BKC tumors, and squamous-cell carcinomas always expressed CK4, CK14, and CK15 and displayed reduced expression of CK7 and CK18 in metaplasia or no expression of CK7 and CK18 in tumors. CK1,10/11 was also often expressed in BKC tumors. In the nonkeratinizing squamous-cell carcinoma, CK7, CK18, CK15, and CK4 were seen. The exclusive expression of CK14 and CK15 and the squamous markers, CK4 and CK1,10/11, in the squamous tumors and in the squamous metaplasias of the DEE-exposed rats underlines the differences between the particleassociated pathogenesis of these lesions and the non-particle associated, nonsquamous lesions (BA hyperplasia, BA metaplasia, BA adenoma, BA carcinoma) found in all treatment groups and the control. We conclude that CK immunostaining is a valuable tool for the differential diagnosis of the epithelial proliferative lesions in the rat lung and may be useful as an intermediate biomarker to investigate precursor lesions in tumors in pulmonary carcinogenicity studies.