Two methods of sperm preparation for in vitro fertilization were compared: the swim-up technique vs. the migration-sedimentation technique. The study comprised fresh semen samples obtained from 25 couples treated in the In Vitro Fertilization Unit. Oocytes aspirated in a single cycle were divided into two groups, each inseminated by sperm prepared by one of these techniques. Motility, degree of motility, and normal morphology were improved by both methods. The improvement was greater when the migration-sedimentation technique was applied. However, fertilization rate was significantly higher after the swim-up technique. In order to clarify this contradiction, an additional group of 26 semen samples was divided and then prepared by the swim-up or migration-sedimentation techniques. Sperm quality was examined up to 72 h after separation. Compared with the swim-up technique, sperm characteristics were better after separation by the migration-sedimentation technique. However, this difference abated after 24 h. The better results of the swim-up technique in the "survival experiment' may explain its improved performance in in vitro fertilization, despite lower separation capacity. Thus, the migration-sedimentation technique is not recommended for sperm preparation in in vitro fertilization.