Human spermatozoa may chemotactically find out the egg by following an increasing gradient of attractant molecules.Although human spermatozoa have been observed to show several of the physiological characteristics of chemotaxis,the chemotactic pattern of movement has not been easy to describe.However,it is apparent that chemotactic cells may be identified while returning to the attractant source.This study characterizes the pattern of movement of human spermatozoa during chemotactic re-orientation towards a progesterone source,which is a physiological attractant candidate.By means of videomicroscopy and image analysis,a chemotactic pattern of movement was identified as the spermatozoon returned towards the source of a chemotactic concentration of progesterone (10 pmol l-1).First,as a continuation of its original path,the spermatozoon swims away from the progesterone source with linear movement and then turns back with a transitional movement that can be characterized by an increased velocity and decreased linearity.This sperm behaviour may help the spermatozoon to re-orient itself towards a progesterone source and may be used to identify the few cells that are undergoing chemotaxis at a given time.