The effect of pregnancy on sarcoma 180 in albino swiss mice.

It has been observed that transplanted tumors in rats and mice sometimes regress during preg nancy (3, 6), the tumor growth usually being more noticeably inhibited in the later stages of preg nancy (1). Spontaneous breast adenocarcinoma in FI hybrids (from C57BL females mated to RIII males) have been reported to regress after par turition or "a little before" parturition (4). This report summarizes the effects of mating1 and pregnancy on the growth of tumors in albino Swiss mice as observed in four experiments that ran from 2 to 4 months each,2 each experiment comprising animals in the same age group. The mice used were virgin females from 8 to 16 weeks old, taken from our own stock colony in which Sarcoma 180 from the Jackson Memorial Labo ratory has been grown since 1951. In the previous 35 passages of this tumor, which has been trans planted every 7 days, spontaneous regression has occurred in 16 per cent of 177 animals.