Previous studies have demonstrated that the in vitro aryl hydrocarbon hydroxylase (AHH) and cytosolic receptor binding bioassays can be utilized to quantitatively estimate the potential in vivo toxicities of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and their mixtures (from municipal fly ash extracts). The study reports the use of the two bioassays in estimating the levels of 3,3',4,4'-tetrachloroazobenzene in several commercial products which contain or are derived from 3,4-dichloroaniline. The AHH induction and receptor binding EC50 values for 3,3',4,4'-tetrachloroazobenzene were 5.60 X 10 to the minus seventh and 5.60 X 10 to the minus sixth M respectively. The levels of 3,3',4,4'-tetrachloroazobenzene (TCAB) in commercial samples of Diuron, Linuron and 3,4-dichloroaniline were determined by gas chromatographic (GC) analysis and the concentrations varied from trace levels to 2200 ug/g. Preliminary bioassays of the commercial compounds and crude extracts (BC) of these products gave contradictory results which did not correlate with the GC analytical data. Presumably there are impurities or coextractives which interfere with the sensitivity of the bioassays. The crude extracts were purified by column chromatography and were assayed using both bioassays.