The DNA damage induced by in vivo administration of: 2,4-dinitroaniline, ortho-toluidine and para-toluidine, was determined using alkaline filter elution of DNA. The target organs for ultimate carcinogens produced in mice appear to be the liver and the kidney. The damage was evident 4 hours after administration of a single dose of ortho- and para-toluidine. The values obtained after treatment with 2,4-dinitroaniline fall in the range of control mice.