The use of cryopreserved aged human oocytes in a diagnostic test of sperm fertilizing ability was evaluated. Oocytes arising from assisted conception cycles and showing no signs of fertilization 48 h post-insemination were cryopreserved by one of two methods. An ultrarapid method using dimethyl sulphoxide gave poor post-thaw results, with only 5/69 (7.2%) oocytes surviving. Oocytes frozen by a slow method using propanediol as the cryoprotectant gave better survival rates (359/594; 60%). Fertilization by donor spermatozoa of these thawed oocytes was poor (15/63; 24%) when the zona pellucida was left intact. To improve this, the zona was enzymatically removed using pronase. These zona-free oocytes were then inseminated with spermatozoa from a fertile donor or from men previously exhibiting fertilization failure in an in-vitro fertilization treatment cycle. The fertilization rate in the patient group (41/91; 45%) was significantly lower than in the donor group (16/18; 89%) (P less than 0.02). There was also a significant (P less than 0.03) reduction in the median number of pronuclei per oocyte (2.9 versus 4.5). These results show that aged oocytes can be effectively cryopreserved to establish a bank for use in a test to identify men with impaired sperm fertilizing capacity.