Abstract: The discovery of uteroglobin resulted from investigations on the biochemical composition of oviductal and uterine secretions of the rabbit and other mammals. These determinations about physiological composition were urgently requested to prepare culture media for research on early mammalian development in vitro. Discovery of significant proteins during the sixties reflected the laboratory skills of that time. Protein characterization was achieved by isolation via Sephadex gels, electrophoresis on polyacrylamide gels, and finally immunoprecipitation using classical polyclonal antibodies. The molecular biology was not yet established. Uteroglobin could be found as the major protein component of rabbit uterine secretion. From the beginning, it was already identified as an unusually small, spheric uterine secretory molecule without any carbohydrates-hence its name. Uteroglobin was the first mammalian protein that turned out to be progesterone-regulated and, at the same time, released in mg amounts actually in one organ compartment. Moreover, uteroglobin and its gene proved to be a reliable model for the description of progesterone/progesterone receptor complex action at the DNA level. After its original observation in the uterus, however, uteroglobin was detected also in several other organs, for example, the epididymis, the seminal vesicle, and the lung. Initially, it could not be found in the blood, which challenged the hypothesis that uteroglobin specifically should operate by local activation rather than by a humoral or endocrine effect. Later, though, the human uteroglobin molecule, isolated from blood filtrate, was used for detailed structural analyses. The rabbit uteroglobin model certainly was beneficial for reproductive biological research. Experimental interference with steroid hormone regulation during preimplantation presented surprising effects, which led to the discovery of the transposition of the implantation window. The uterine secretion protein patterns, in particular the uteroglobin fraction and the β-glycoprotein fraction, served as decisive marker profiles to identify the biological stage of the intrauterine microenvironment during preimplantation. This diagnostic procedure, using only protein parameters, enabled us to precisely predict the receptive stage of the endometrium for donated blastocysts to achieve implantation successfully.