The sperm penetration test (SPT) in cervical mucus (CM) is an important test in screening for sperm-CM incompatibility. For a routine SPT, a sufficient amount of preovulatory CM is necessary, but the amount of human CM is often inadequate. We examined estrous bovine CM as a substitute for human CM in the SPT. Preovulatory human CM and estrous bovine CM were collected and stored frozen at -20 degrees C until use. After being thawed at room temperature, the two CM samples were compared as to their pH, spinnbarkeit and ferning patterns, and it was found that they are quite similar. In Kremer's method of SPT, sperm penetration distance, density and motility were similar in human and bovine CM for 12 hours, but thereafter sperm motility became much lower in bovine CM than in human CM. The results of SPT with spermatozoa from patients with oligozoospermia were quite similar in both CMs. When spermatozoa pretreated with antisperm antibody were used for SPT, sperm penetration was completely blocked in human CM, but no inhibition was seen in bovine CM. From these results, we conclude that bovine CM can be substituted for human CM for some, but not all, purposes.