Bull. Environm. Contain. Toxicol. 30, 80-83 (1983) Quantitative Structure-Toxicity Relationship of Chlorinated Compounds: A Molecular Connectivity Investigation Aleksandar Sabljic Laboratory of Chemical Physics, National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases, National Institutes of Health, Bldg. 2, Room B2-0!, Bethesda, MD 20205, and Theoretical Chemistry Group, "Rudjer Bo~kovid" Institute, P.O. Box 1016, 41001 Zagreb, Croatia, Yugoslavia An estimated 70 000 chemical compounds are currently in common use and their number grows by about i000 each year. Because potential risk is present for each synthesized chemical, their toxicity testing is required. In addition, water is polluted by chlorinated compounds from industrial discharges, from agricultural use of pesticides, and from water chlorination in large cooling systems (e.g. ~uclear reactors). Almost all these compounds are highly toxic (MULLER 1976) and some of them are known or suspected carcinogens (TOMATIS et al. 1978). Because of their widespread use, persistence, concentration in food chains, and toxicological properties, they constitute amajor group of potential environmental hazards to living species, including man. Thus, there is a need for a simple and accurate method for the preliminary estimation and ranking of the potentially hazardous chemicals, which will enable us to create an adequate priority list for testing them. Several physico-chemical properties have been found to correlate satisfactory with various acute toxicity parameters and thus far th# octanol/water partition coefficient is the most successful (K~NEMANN 1981 ). Here, an alternative approach for toxicity prediction of chlorinated compounds will be presented. Its simple applicability and purely nonempirical nature make this method preferable to physico-chemical properties, which necessitate experimental deter- minations and/or reference tables. The method is known as molecular connectivity. It is based on molecular topology. Several extensiv 9 reviews (BALABAN et al. 1980, SABLJIC & TRINAJSTIC 1981, TRINAJSTIC 1982) on the theory and calculation of topological indices, as well as their application in structure-property and structure- -activity relationships, have been published recently, which can be consulted for a detail description. Molecular connectivity indices have been found to correlate with toxic effects of alcohols and their derivatives (KIER & HALL 1982) and with the toxicity of selected nitrogenous heterocyclic compounds (SCHULTZ et al. 1982). This work presents our continuous interest in the fate and behaviour of chlorinated compounds in the environment and in the applicabi- lity of the topological indices in ecotoxicological studies (SABLJIC & PROTIC 1982). The acute toxicity data used in this study are from the work of HEITMULLER et al. (1981). These data were obtained under uniform conditions, using methods recommended by the U.S. EPA. 0007-4861/83/0030-0080 $01.00 9 1983 Springer-Verlag New York Inc.