The Relation Between Pseudopregnancy and the Chemical Induction by Four Carcinogens of Mammary and Ovarian Tumours in BALB/c Mice

did not occur in BALB/c virgin or lobectomised female mice following skin application of 20-methylcholanthrene (MC) for a limited period of 12 weeks. Virgin BALB/c mice, even when caged 5 in a box, rarely undergo pseudopregnancy (Caschera, 1960) and do so less frequently when the olfactory lobes have been removed. By rendering BALB/c mice artificially pseudopregnant the incidence of mammary tumours following MC treatment was 44 per cent. Marked differences were observed in the structure of the unaffected breasts, uteri and ovaries in non-pseudopregnant and pseudopregnant mice, and thus it was concluded that the hormonal conditions of pseudopregnancy were requisite for the development of mammary cancer following limited chemical treatment of the strain. Ovarian tumours did not occur in any of the groups. Bonser (1958) observed that a limited dose by skin application of the four carcinogens 9,10-dimethyl-1,2-benzanthracene (DMBA), MC, 1,2: 5,6-dibenzanthracene (DBA) and 3,4-benzopyrene (BP) induced mammary tumours in virgin IF mice; Biancifiori, Bonser and Caschera (1961) obtained a similar result i