Pregnancy, like most biologic phenomena, involves the action of cytokines. These proteins have a short half-life and are believed to exert their effect close to their site of production, where diagnostic tests cannot be easily performed. Here we show that the cytokine content in the maternal serum reflects cytokine production and secretion from maternal spleen cells, which also correlates with production from decidual cells. We show that GM-CSF, IL-3, and IL-10 are present in the serum at specific time intervals during the first half of murine pregnancy, which correlates with their production from maternal spleen cells. Purified GM-CSF and IL-3 from spleen-cell-culture supernatants are biologically active molecules, able to stimulate placental-cell proliferation. Furthermore, TNF-alpha, which has been identified in many cases of fetal rejection as well as in labor, is shown to be naturally produced during the second half of pregnancy. Additionally, within the limits of the sensitivity of the technique we have used, the detection of IL-4 and the absence of detectable levels of IL-2 in the maternal serum strongly comforts the hypothesis that pregnancy is a Th2-dependent phenomenon. The results presented in this paper show that the cytokine profile during pregnancy can be monitored by simple blood tests, which may be of relevance both in the followup of a physiological human pregnancy and to the diagnosis of recurrent abortions due to cytokine imbalance.