Mutation Research, 106 (1982) 91-99 91 Elsevier Biomedical Press Decreased viscosity of rat-liver DNA treated by Y-methyl-4-dimethylaminoazobenzene,-- -- ---- detected with a new viscometric approach S. Parodi 1, C. Balbi 2, M. Taningher 1, M. Pala 2, p. Russo 2, M.L. Abelmoschi 2 and L. Santi 1 1 Institute of Oncology, University of Genoa and 2 Istituto Seientifico per Io Studio e la Cura dei Tumori, Viale Benedetto XV, 16132-Genoa (Italy) (Received 1 December 1981) (Revision received 26 March 1982) (Accepted 22 April 1982) Summary DNA damage induced in vivo by Y-methyl-4-dimethylaminoazobenzene (YCH3DAB) was investigated with 2 differently sensitive techniques: the alkaline elution assay and the viscometric measurement of DNA damage. YCH3DAB appeared to be falsely negative with the alkaline elution assay, whereas with the viscometric approach, which is about 30-50 times more sensitive, it appeared positive, and the DNA damage was dose-dependent. Data by many authors indicate that Y-methyl-4-dimethylaminoazobenzene (YCH3DAB) is carcinogenic in small rodents. When administered in the diet it induced hepatocellular carcinomas and cholangiosarcomas in rats (Goldfarb, 1973; Boyd et al., 1978b). When administered subcutaneously in Swiss mice, in the first 5 days of life, it induced liver tumours in males and lung tumours in both sexes (Roe et al., 1971). The following data concerning its detectability, in vitro, in short-term tests is available from the literature. The mutagenicity of 3'CH3DAB was de- termined in the standard Salmonella test with or without the liver-microsome activating system; the data obtained were contradictory. The compound was posi- tive, but the number of revertants per #mole was relatively low in 3 studies (McCann et al., 1975; Yahagi et al., 1975; Simmon, 1979a). In another investigation with the same bacterial system, YCH3DAB gave no evidence of mutagenicity, possibly owing This work was supported by the Italian National Council of Research (Progetto Finalizzato 'Controllo della Crescita Neoplastica'; contracts No. 80.01608.96 and No. 80.01650.96). 0027-5107/82/0000-0000/$02.75 © Elsevier Biomedical Press