Percoll treatment of bovine frozen/thawed spermatozoa was identified as the cause of low cleavage and blastocyst rates in our in vitro embryo production system. Percoll treatment of spermatozoa yielded very high, stable and repeatable results for many months of cleavage and blastocyst rates of 83 +/- 5% (+/-SD) and 33 +/- 10%, respectively. This was followed by a period with significantly lower cleavage and blastocyst rates of 60 +/- 9% and 14 +/- 5%, respectively. In the last period, the results became even lower, with cleavage and blastocyst rates of 32 +/- 5% and 5 +/- 3%, respectively. These results were significantly different from those of the 2 previous periods. We were able to correlate precisely these changes in outcomes to the introduction of new Percoll batches. When compared with washed spermatazoa and under otherwise identical circumstances, the cleavage and blastocyst rates from spermatozoa treated with Percoll increased significantly from 33 to 73% and from 4 to 26%, respectively. We suggest that this adverse effect of Percoll is not due to Percoll particles per se, but may be ascribed to the effect of unbound polyvinylpyrrolidone (PVP) in the Percoll.