The hypothesis was tested that 7-sulfooxymethylbenz[a]anthracene (7-SBA) is an ultimate electrophilic and carcinogenic form of 7-hydroxymethylbenz[a]anthracene. In conformity with this hypothesis, 7-SBA was more carcinogenic than 7-HBA in inducing sarcomas at the site of repeated subcutaneous injection. These metabolites were individually administered to female Sprague–Dawley rats, beginning at 30 days of age, in 0.2 μmol doses given three times each week for 20 doses. One year after the first injection of 7-SBA, seven of thirteen female Sprague–Dawley rats had developed sarcomas. 7-HBA, on the other hand, had induced sarcomas at the site of injection in only two of twelve rats. No tumors developed either in the control group given sesame oil:DMSO only or in the untreated control group. It would appear from the results summarized here that the search for an ultimate electrophilic and carcinogenic form of 7-HBA has been successful.