The effect of immunization of boars with boar sperm or seminal plasma on the sperm quality was investigated. Four 15-month-old boars, seronegative for sperm antibodies, were immunized intramuscularly with washed, sonicated sperm cells mixed with Freund's complete adjuvant at weeks 0, 2 and 15 and subsequently with seminal plasma at weeks 37 and 39. Semen and sera were collected weekly and examined for sperm motility and agglutination and for sperm antibody titer, respectively. High sperm antibodies were detected in serum and seminal plasma samples of all four boars, and the antibody titers inversely correlated with sperm motility (r= -0.91, P<0.001). Sperm agglutination along with sperm antibodies were observed by the subsequent immunization with seminal plasma in the four boars. There was no significant difference in the results between boars (P>0.05). It is suggested that the reduced sperm motility and the presence of sperm agglutination may be associated with the antibodies produced against sperm and seminal plasma proteins.