Five methods of water purification for culture media preparation have been described. The quality of media thus prepared has been assessed retrospectively by relating to the fertilization and pregnancy rates of a total of 646 treatment cycles in an in-vitro fertilization and embryo transfer (IVF-ET) programme. All media for human use was subjected to quality control assessment in a mouse embryo culture system. Although no differences were seen in the fertilization rates or the proportion of women proceeding to embryo transfer, significant differences were seen in the pregnancy rates. The highest pregnancy rates were obtained using rainwater distilled six times (22/100, 22%) or eight times (40/184, 21.7%), and water polished using a Milli-Q water purifying system (41/181, 22.7%). Significantly lower pregnancy rates were obtained when the water was purified using an alternative Milli-Q system (9/89, 10.1%) or dam water distilled six times (6/63, 9.5%). These findings may reflect contamination of local dam water supplies but demonstrate the difficulty in purifying water for the preparation of culture medium.