Use of a medium-term liver focus bioassay to assess the hepatocarcinogenicity of 1,2,4,5-tetrachlorobenzene and 1,4-dichlorobenzene Daniel L. Gustafson a, *, Amy L. Coulson a , Lixin Feng a , Wendy A. Pott a , Russell S. Thomas a , Laura S. Chubb a , Shakil A. Saghir a,1 , Stephen A. Benjamin b , Raymond S.H. Yang a a Department of Environmental Health, Center for Environmental Toxicology & Technology, Colorado State University, Fort Collins, CO 80523-1680, USA b Department of Pathology, Center for Environmental Toxicology & Technology, Colorado State University, Fort Collins, CO 80523-1680, USA Received 11 December 1997; received in revised form 20 February 1998; accepted 20 February 1998 Abstract 1,2,4,5-Tetrachlorobenzene (TeCB) and 1,4-dichlorobenzene (DCB) are important environmental contaminants that have been used extensively for a variety of industrial applications. Limited data are available in the literature regarding the carcinogenicity of TeCB. DCB has been shown to cause renal adenocarcinomas in rats and hepatic adenomas and carcinomas in mice at high doses in a 2-year study. In the studies presented here, we report that TeCB can promote the formation of preneoplastic foci and DCB cannot in a medium-term initiation/promotion assay. These results suggest that TeCB is a liver tumor promoter and that DCB is not at fairly low doses (0.1 and 0.4 mmol/kg per day).  1998 Elsevier Science Ireland Ltd. All rights reserved Keywords: 1,2,4,5-Tetrachlorobenzene; 1,4-Dichlorobenzene; Hepatocarcinogenicity; Medium-term bioassay 1. Introduction The chlorobenzenes are a small class of cyclic aro- matic hydrocarbons formed by the chlorine substitu- tion of a single benzene ring. A total of 12 different isomers of chlorobenzene can be formed which have a wide range of physio-chemico properties and com- mercial applications [1]. 1,2,4,5-Tetrachlorobenzene (TeCB) is used commercially as an intermediate in the production of herbicides, insecticides, fungicides, disinfectants and preservatives [1] and has been found as a contaminant in fish in the Great Lakes [2] and in human adipose tissue [3]. 1,4-Dichlorobenzene (DCB) is used as a moth repellent, deodorant and as an inter- mediate in the production of dyestuffs, insecticides, pharmaceuticals and organic chemicals [1]. DCB has been shown to be present in avian [4] and human [4,5] tissues in response to environmental exposure. Cancer Letters 129 (1998) 39–44 0304-3835/98/$19.00  1998 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3835(98)00078-0 * Corresponding author. CETT, Foothills Campus, Colorado State University, Fort Collins, CO 80523-1680, USA. Tel.: +1 970 4912192; fax: +1 970 4918304; e-mail: dangus@cvmbs.colostate.edu 1 Present address: Beak International Corporation, Toronto, Ontario, Canada.