A number of sites in the state of Missouri have been contaminated with polychlorinated dibenzodioxins and dibenzofurans as a result of improper waste‐oil application for dust control. In addition to these compounds, relatively high levels of 1,2,4,5,7,8‐hexachloro(9H)xanthene (1,2,4,5,7,8‐HCX), a by‐product of hexachlorophene manufacture, were also detected. Unlike the dibenzodioxins and dibenzofurans, no animal toxicity data are available on the chlorinated xanthenes. In view of the potential importance of this novel class of environmental contaminants, studies were conducted to examine the acute oral toxicity in guinea pigs and in vitro “dioxin‐like” activity of 1,2,4,5,7,8‐HCX. Animals administered a single oral dose of 0.02, 0.1, 0.5 or 2.5 mg 1,2,4,5,7,8‐HCX/kg in corn oil and sacrificed 42 d later exhibited no treatment‐related pathology. Guinea pigs given 12.5 mg/kg displayed mild to moderate distension and histologically observable subserosal edema of the urinary bladder, in addition to mild fatty vacuolization of pancreatic acinar cells. These alterations were considered to be of minimal toxicological significance. No compound‐ or dose‐related mortality, body weight loss, or organ weight changes were noted at any dose level. Results using an in vitro bioassay for “dioxin‐like” activity confirmed preliminary data suggesting that 1,2,4,5,7,8‐HCX is about 106times less potent than 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (2,3,7,8‐TCDD) in this assay system. These findings indicate that 1,2,4,5,7,8‐HCX may represent a relatively low environmental hazard compared to 2,3,7,8‐TCDD.