The histological lesions, chromosomal characteristics and transplantability of six erionite-induced and seven UICC chrysotile-induced rat mesotheliomas are compared. The tumors were of four types: tubulopapillary, fibrosarcomatous, mixed fibrosarcomatous and tubulopapillary and mixed fibrosarcomatous and chondrosarcomatous. Cells lines derived from these displayed heterogeneous chromosome anomalies, but none were unique either to chrysotile or erionite treatment. Five of seven erionite-induced and four of six UICC chrysotile-induced tumors had various anomalies of chromosome no. 1. When seven cell lines were transplanted into syngeneic rats, all produced tumors that were pathologically similar to the original tumor regardless of the route of injection. Cytogenetically, the cell lines derived from tumors after intrapleural transplantation resembled the injected cell line.