Recent studies have indicated that human spermatozoa respond to follicular fluid by attraction to chemotactic factor(s) in the fluid, accompanied by enhancement of motility and ultimately hyperactivation. In this study, we quantified the sperm response. We exposed spermatozoa to a gradient of a chemotactically active fraction of follicular fluid (denoted as 'the attractant') and separated the spermatozoa that accumulated in the attractant and those that did not. We thus obtained two subpopulations: one enriched with chemotactically responsive spermatozoa, and one deficient in such spermatozoa. The fraction of the responsive spermatozoa out of the total sperm population was 2-12% at any measured time point. With time, the responsive spermatozoa lost their ability to be attracted, while such activity was gradually acquired by the subpopulation originally deficient in responsive spermatozoa. These results indicate that the identity of responsive spermatozoa is continuously changing. If the in vitro results are representative of the physiological conditions in vivo, they imply that the role of sperm chemotaxis combined with enhanced motility may be to select capacitated spermatozoa and bring them to the egg. Such a mechanism may, over an extended period of time, increase the prospect that an egg will meet capacitated spermatozoa as soon as it ovulates.