Human and mouse oocytes were cryopreserved by a slow freeze, rapid thaw method, using propanediol (PROH) as the cryoprotectant. A simulated cryopreservation was also included in the study to detect the level of damage attributable to the PROH alone. Comparison of the mouse and human oocytes cryopreserved by the same method showed opposing results, with a poor morphological survival rate of 4% observed for mouse oocytes and a subsequent normal fertilization rate of 0%. In 171 cryopreserved human oocytes a higher survival rate of 64% was achieved, and this showed more similarity to the mouse pronuclear oocytes survival of 53%. A comparison of human oocytes, cryopreserved within the cumulus and denuded of cumulus and corona prior to cryopreservation, demonstrated a higher survival rate in the denuded oocytes of 69% compared to 48%. A delay prior to cryopreservation of 1 or > or = 2 days had no effect on the immediate survival of oocytes, but culture for a further 24 h after thawing reduced survival, with the day 1 oocytes exhibiting the most dramatic reduction in survival (28%). Using a lectin binding method, abundant cortical granules were observed in all cryopreserved oocytes analysed. The meiotic spindle and chromosomes were examined in cryopreserved oocytes using fluorescence microscopy and 60% of the surviving oocytes had a normal spindle and chromosome configuration.