Sperm-mediated gene transfer (SMGT) can provide the most efficient and cost effective technique to generate transgenic animals. We investigated various aspects of sperm/foreign DNA interaction 'in vitro.' The rate of DNA internalization was high leading to around 104 plasmids present in sperm nuclei. The DNA treated sperm showed a significant 2-fold decrease in motility and a 5-fold decrease in progressive motility caused by removal of seminal plasma during washing. No additional effects of DNA treatment on motility and viability of sperm were detected. Increased damage of genomic DNA caused by sperm washing and DNA incubation was detected by Comet assay. No green fluorescent protein positive transgenic embryos were detected after testing 105 fertilized embryos (>= 2 cells) produced by 18 sows inseminated with pEGFP treated sperm from 3 different boars. It was found that a significantly higher fertilization rate (84%) was obtained with DNA treated semen. It is suggested that failure of SMGT might result from activation of natural defences against invasion of foreign DNA.