Primordial germ cells (PGCs) are the stem cell precursors of the germ line. Several growth factors contribute to enlarging the PGC population by acting as mitogens, survival factors or both. Interleukin-2 (IL-2) has a growth-promoting activity for T and B-lymphocytes, but its role in PGCs had not yet been studied. Here, we show that PGCs isolated from 10.5, 11.5 and 12.5 day postcoitum (dpc) mouse embryos constitutively express the three subunits (alpha, beta and gamma) of the IL-2 receptor (IL-2R). In contrast, IL-2 mRNA was not detected in these cells. However, the addition of recombinant IL-2 to the culture medium increased the number of PGCs in vitro via a mitogenic effect, as indicated by bromodeoxyuridine incorporation assays. Neutralization of the IL-2 receptor using anti-IL-2R subunit antibodies inhibited this IL-2-mediated proliferative effect on PGCs from 11.5 dpc embryos. Together, these data are indicative of a paracrine effect of IL-2 on PGC proliferation. In this regard, we also compared the effect of IL-2 with other compounds such as basic fibroblast growth factor (bFGF), steel factor, leukemia inhibitory factor and forskolin, and found that the degree of proliferation induced by IL-2 was similar to that induced by bFGF and forskolin. These observations support the notion that similar patterns of molecular signaling may underlie the developmental pathways of hematopoietic and germ stem cell precursors.