The response of human sperm motility and hyperactivation to the stimulant 2'-deoxyadenosine (2'-DEA) was studied in vitro using computer-assisted sperm motion analysis. A total of 20 randomly selected individuals with normal sperm counts as defined by the World Health Organization were chosen and their migration-separated spermatozoa exposed to a range (0.1-10.0 mM) of concentrations of 2'-DEA. The straight line velocity (VSL) was increased above control values only at 0.1 mM, while the curvilinear velocity (VCL) and lateral head displacement (ALH) were increased significantly at all concentrations. Linearity of progression (LIN), on the other hand, declined with increasing concentration of 2'-DEA. These changes were related to a significant increase in the number of spermatozoa exhibiting hyperactive-like motion. There was, however, considerable intra-individual variability in the response to 2'-DEA. In some individuals VCL and ALH exhibited little or no response to 2'-DEA, whilst in others an increase above the control of 50-55% occurred. The maximum response for VCL and ALH occurred at 2.5 mM 2'-DEA. Individuals showed greater variability in the percentage of spermatozoa exhibiting hyperactivity in response to 2'-DEA, with increases ranging from 76 to 948% of the control value, although the maximum response was also most commonly seen at 2.5 mM 2'-DEA. The diversity of response to 2'-DEA emphasizes the importance of tailoring doses to the individual rather than employing one concentration for all. Further tests on a subgroup of the individuals examined the longevity of spermatozoa in response to 24 h of continued exposure to 2'-DEA.(ABSTRACT TRUNCATED AT 250 WORDS)