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Using zebrafish, Danio rerio, initial pioneering work in the 1960s revealed carcinogen responsiveness of fish, yet very few subsequent tumorigenesis investigations have utilized this species. We exposed embryos (60 hours postfertilization) and fry (3 week posthatch) to 7,12-dimethylbenz[a]anthracene (DMBA) by immersion in aqueous solutions for 24 hours, at concentrations of 0-1 or 0-5 ppm (mg/L), respectively. Juvenile zebrafish 2 months posthatch were fed a diet containing 0-1,000 ppm DMBA for 4 months. Fish were sampled for histologic evaluation at 7-12 months after the onset of carcinogen treatment. Fry were most responsive to DMBA and showed the widest diversity of target tissues and histologic types of neoplasia, having several types of epithelial, mesenchymal, and neural neoplasia. The principal target tissues for carcinogenic response were liver following embryo or fry exposure, with gill and blood …