When day-3 mouse embryos were cultured in a simple medium supplemented with uterine fluids of mice autopsied on day 4 of pregnancy, 48 h after administration of oestradiol, or during pro-oestrus, the percentage of blastocysts hatching from the zona pellucida was significantly greater than in unsupplemented medium. In the presence of uterine fluids recovered during delayed implantation this stimulation of blastocyst hatching was not observed. When the culture medium was supplemented with dialysed uterine flushings containing 20 or 30 microgram protein/ml, both 'day 4' and 'delay' uterine proteins were equally effective in enhancing hatching frequency (P less than 0.05). The results suggested that 'delay' uterine fluids may contain a dialysable inhibitor of blastocyst activity. The putative inhibitor was not effective in the presence of serum, since uterine fluids recovered both on day 4 of pregnancy and during delayed implantation significantly increased the size attained by blastocyst outgrowths in the presence of foetal calf serum (P less than 0.01). The percentage of blastocysts exhibitng giant cell transformation and outgrowth was also increased (P less than 0.02) by these uterine fluids when the concentration of FCS in the medium was minimal (0.25%).