The influence of cell-free extract of mouse fibrosarcoma and normal fibroblast on the early development and organogenesis of mouse embryos, and on the histology of non-pregnant and pregnant uteri was studied in vivo. Both normal and tumour cell extracts inhibited normal mouse embryogenesis. However, the effects of tumour extract on mouse embryogenesis was far more drastic and significant. Despite such changes, no basic alteration of the morphology of the embryonic cells was seen. Observations on the uterine histology indicated extensive deciduation. Pyknosis followed by degeneration of the endometrium was the main effect of tumour extract on the pregnant mouse uterus. Such inhibition of embryogenesis and degenerative effect on the mouse uterus by tumour cell extract might be due to general degradative effects of the tumour constituents.