The APC^Min mutation causes development of intestinal and mammary tumors (Moser et al., Eur. J. Cancer 31: A7-8, 1061-1064, 1995) in mice. We tested the genetic relationship of modifiers of APC^Min effects in the two organs, using the OcB/Dem recombinant congenic strains, each of which carries a different subset of about 12.5 percent of C57BL/10 genes on the genetic background of the strain O20 (Stassen et al., Mammalian Genome 7: 55-58: 1996). The relative susceptibility of the different OcB strains with APC^Min mutation to intestinal (cover panel 1) and mammary (cover panel 2) tumors appeared to be inverse (cover panel 3), suggesting that subsets of modifiers of the APC^Min mutation have opposite effects in the intestine and in the mammary gland. The proportion of mammary adenoacanthomas (cover panel 2) and adenocarcinomas also differed between OcB strains. The data indicate that the allelic effects of a significant set of susceptibility genes/modifiers are different, or even opposite, in different organs. For details, see the article by Czarnomska et al. on p. 4533 of this issue.