Addition of serum to culture medium during washing of sperm was shown to significantly improve sperm motility upon subsequent 24-hour incubation, distinct from the beneficial effects of adding serum after washing. Commercial sources of human and bovine albumen tested did not preserve motility as well as serum, but addition to serum showed that the lesser effect was probably due to toxic contaminants. One of the most commonly available disposable plastic syringes was found to be markedly toxic to sperm. The materials found to have sperm toxicity also manifested toxicity to mouse embryos. These findings show that a similar degree of caution is advisable in testing of materials contacting sperm in vitro to that which has become standard practice for culture of human embryos.