Male RFM mice were given localized chest exposures of 750-3000 R of x-rays, with and without injection of 400 mg of WR-2721 (S-2-[3-aminopropylamino]ethylphos-phorothioic acid) before exposure, and were killed 11 months later for analysis of lung tumor patterns. By a technique of clearing and staining, lung tumors as small as 0.1 mm in diameter could be detected. Essentially no mortality occurred over this 11-month interval, so the data could be interpreted without the complication of premature removal; i.e., all mice were available for analysis. The incidence of lung tumors (and the mean number of tumors per mouse) was directly related to exposure over the range of 750-1500 R and then declined to control levels over the range of 1750-3000 R. Tumors occurred randomly among the members of an exposed group and were typical adenomas with various degress of invasiveness. Injection of WR-2721 before exposure did not affect the incidence between 750 and 1250 R but was associated with a peak at 1250 R, as opposed to 1500 R in the nontreated mice; and the incidence in drug-treated mice was lower than that in nontreated mice at all higher exposures. The tracheobronchial lymph nodes were protected by WR-2721, and this protection may account for at least part of the effect of the drug on tumor incidence.

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