The uterus and placenta of the mouse and rat produce a member of the prolactin (PRL) family referred to as decidual/trophoblast PRL-related protein (d/tPRP). This cytokine/hormone has been hypothesized to regulate decidual cell activities needed for the establishment and maintenance of gestation. An alkaline phosphatase (AP)-tagging strategy was used to identify d/tPRP target cells. AP-d/tPRP bound to virtually all cells and tissues to which it was exposed, consistent with our earlier evidence that d/tPRP binds to heparin-containing molecules. Moreover, we found that co-incubation with heparin or pretreatment with heparitinase greatly decreased the binding of AP-d/tPRP to tissue sections. In addition, we observed that the AP-d/tPRP probe bound to the surface of Chinese hamster ovary (CHO) cells but not to heparan sulfate-deficient CHO-pgsD-677 cells. Potential unique non-heparin d/tPRP binding sites within mouse and rat uteroplacental tissues were identified by consecutively incubating sections with AP-d/tPRP followed by heparin. This strategy led to the identification of d/tPRP target cells associated with the uterus and the labyrinth zone of the chorioallantoic placenta. Within the uterus, d/tPRP specifically bound to eosinophils. d/tPRP-binding and eosinophil peroxidase activity were co-localized and showed similar patterns of distribution during the estrous cycle, pregnancy, and following hormonal manipulation. d/tPRP interactions with eosinophils were further demonstrated in the lung and intestine, with eosinophils isolated from the peritoneum, and in mice with generalized tissue eosinophilia. Collectively, these findings suggest that intercellular d/tPRP targeting is mediated through associations with heparin-containing molecules which help direct d/tPRP to specific interactions with eosinophils within the uterus and with the labyrinthine compartment of the chorioallantoic placenta.