The present study was designed to investigate whether autoantibodies to external domains of the sperm plasma membrane affect the movement of normal motile spermatozoa. Eight sera and 20 seminal plasma samples containing high levels of anti-sperm antibodies as well as antibodies eluted from the sperm fraction of 19 autoimmune ejaculates were incubated with donor's motile spermatozoa, obtained by swim-up migration in Tyrode's solution. Sperm movement was analysed using 1 s exposure microphotography when greater than 70% of the spermatozoa were coated with antibodies (after 30-90 min of incubation). At least 50 tracks of progressively motile spermatozoa were analysed in order to obtain the mean values of the amplitude of lateral head displacement (ALH) and the velocity of progression (VSL). Serum antibodies and sperm eluted antibodies had quite consistent but opposite effects on sperm movement; serum antibodies increased ALH and decreased VSL whereas eluted antibodies decreased ALH and increased VSL. Seminal antibodies did not affect these two parameters significantly. Furthermore, seminal antibodies and sperm eluted antibodies obtained from the same ejaculates had distinct effects on ALH and/or VSL. This diversity was apparently not linked to antibody isotype or localization on the sperm membrane; it might be due to differences in the composition of the extracellular media. These results suggest a dynamic effect of anti-sperm antibodies on sperm movement, a possibility that merits further investigation.