Experiments were carried out in which semen samples were diluted 1:1 with Tyrode's solution or with their own seminal plasma (obtained by centrifuging another semen aliquot) as a control. Each experiment consisted of a paired comparison of these two sperm suspensions, using a quantitative cervical mucus penetration test with aliquots of the same mucus sample. Videomicrography was used to measure the swimming speeds of spermatozoa in the semen and in the mucus. The spermatozoa swam faster in Tyrode's diluted seminal plasma than in whole seminal plasma, but their swimming speeds in cervical mucus were similar after mucus penetration. Significantly more of the collisions between spermatozoa and the mucus resulted in successful penetration in tests where the sperm were suspended in whole seminal plasma than in tests where they were suspended in Tyrode's diluted seminal plasma. These observations indicate that components of the seminal plasma are important for efficient entry of human spermatozoa into cervical mucus.