Midkine (MK) is known to be a member of a new family of heparin-binding growth/differentiation factors, together with pleiotrophin, and to be quite rich in bovine follicular fluid. To examine whether treatment with MK during in vitro maturation (IVM) of bovine granulosa-enclosed oocytes affects their nuclear maturation and postfertilization development to the blastocyst stage, bovine granulosa-enclosed oocytes obtained from slaughterhouse-derived ovaries were cultured for 24 h in IVM medium without (control) or with various concentrations (1-500 ng/ml) of MK, followed by in vitro fertilization (IVF) and culturing. Although the MK treatment during IVM did not affect the rate of nuclear maturation or the postfertilization cleavage of oocytes, MK at greater than or equal to 10 ng/ml significantly (P < 0.05) increased the blastocyst yields per tested and per cleaved oocyte compared with the case of the control. Next, the effects of various glycosaminoglycans (heparin, heparan sulfate, chondroitin sulfate A and C, and hyaluronic acid) preincubated with MK at 50 ng/ml were examined. The enhancing activity of MK was completely suppressed by heparin at 600 ng/ml but not by the other compounds. The effects of MK during IVM were also tested on oocytes freed from granulosa cells (GCs), When the denuded oocytes were cultured in IVM medium, no blastocyst formation after IVF was observed, regardless of MK supplementation. However, coculture of the denuded oocytes with isolated GC pellets enhanced the cleavage rates and the blastocyst yield, and these effects were more pronounced with MK supplementation. These results indicate that the presence of MK during IVM of bovine granulosa-enclosed oocytes can enhance their developmental competence to the blastocyst stage after IVF and suggest that the enhancing effects might be mainly mediated by GCs.