We demonstrate the presence of complement factor B (Bf) and complement C3 in uterine luminal fluid collected from estrogen-stimulated immature and adult female mice. We examined the synthesis and secretion of these two proteins in mouse endometrium at various stages of the natural estrous cycle and during the pregnancy period. The mRNA levels of these two proteins increased markedly in proestrus and estrus and declined sharply in metestrus to an undetectable level. The Bf mRNA remained undetectable, whereas a readily detectable C3 mRNA level reappeared, in diestrus. Meanwhile, these two proteins were immunolocalized to the apical cytoplasm of glandular and luminal epithelial cells of the endometrium during the estrous cycle. Administration of an estrogenic steroid to immature or ovariectomized adult mice markedly stimulated the expression of Bf, C3, and their RNA messages in the endometrium, whereas injection of progesterone alone to ovariectomized animals did not stimulate their expression. Expression of C3 was remarkably enhanced, whereas that of Bf changed only slightly, after injection of combined estrogen and progesterone to ovariectomized animals. In pregnant mice (Day [D] 1 = day of vaginal plug), Bf mRNA was at a high level on D1 and D2, dropped to an almost undetectable level from D3 to D8, and then increased to a low level thereafter until delivery. The C3 mRNA was at a high level on D1, dropped on D2 to an almost undetectable level from D3 to D9, increased to a very high level from D10 to D18, and then declined sharply before delivery. Immunohistochemical patterns of both proteins in the endometrium during preimplantation were positively correlated with changes in their mRNA levels.