The influence of preimmunization with a rectal extract on X-irradiation rectal carcinogenesis in the rectum was investigated in male 4-7-week-old A/HeJ mice. They were given 20 Gy per week for a total 40 Gy in the pelvic region with or without 2 prior injections of rectal extract emulsified with complete Freund's adjuvant. Rectal adenocarcinomas were observed in 21 of 34 (62%) immunized animals and in 4 of 22 (18%) non-immunized mice 32 weeks after the last X-irradiation (P<0.01). The total incidences of adenocarcinomas at the 84 week time point were 25/45 (56%) and 12/36 (33%), respectively (P<0.05).

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