Colony stimulating factor-1 (CSF-1) is a glycoprotein growth factor required for the proliferation and differentiation of mononuclear phagocytic cells (reviewed in ref. 1). A 10,000-fold elevation of mouse uterine CSF-1 during pregnancy, suggested by studies of the bone marrow colony stimulating activity of uterine extracts^2,3, was recently demonstrated by radioimmunoassay (RIA)⁴. This increase and the observations that placenta and choriocarcinoma cell lines express c-fms messenger RNA and the c-fms proto oncogene product (CSF-1 receptor⁵) respectively^6,7, suggest an additional role for CSF-1 in pregnancy. We now show that uterine CSF-1 concentration is regulated by the synergistic action of female sex steroids, oestradiol-17β(E₂) and progesterone (P) and that the elevation in CSF-1 concentration can be attributed to the preferential expression of an alternatively spliced CSF-1 mRNA by uterine glandular epithelial cells. These findings indicate that CSF-1, under hormonal influence, plays a role in placental development and function and that steroid hormones may regulate developmental processes via their effects on the expression of tissue-specific growth factors.