The developmentally regulated expression of c-mos oncogene has led to the speculation that this gene may be involved in gametogenesis and early development. To directly test this possibility, we have used an electric field mediated transfer method to introduce an antibody against the c-mos gene product into living immature mouse oocytes. Control oocytes exposed to the electric field without antibody, non-immune IgG, or c-mos antibody pre-absorbed with the mos peptide underwent normal germinal vesicle breakdown (GVBD) (90%) and formed a polar body by 8 h. Oocytes transferred with antibody against c-mos product underwent GVBD, and chromosome condensation as judged by Hoechst 33258 staining. However, antibody transferred oocytes did not form a polar body. Confocal fluorescence microscopy using antibodies against tubulin demonstrated that 90% of the oocytes that received antibody against c-mos did not assemble a meiotic spindle. In a few instances an abnormal spindle-like structure did form. Electron microscopy confirmed that the nuclear envelope disassembled and revealed many microtubules in a disorganized manner. Western blot analysis showed the presence of p39c-mos in mouse mature oocytes, spermatocytes and granulosa cells. These results suggest a role for c-mos in regulating the assembly and/or function of the spindle during meiotic division in murine.