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We demonstrated anticomplementary activity on once-washed human sperm, and in normal and vasectomized seminal plasmas. It was demonstrated to be a normal component of human semen. The origin of the activity is proposed to be the seminal plasma with sperm adsorption of activity. The properties of the seminal anticomplementary factor were characterized further, and the molecular size was shown to be less than 3500 daltons. Reduced anticomplementary activity was found to be associated significantly with abnormal semen profiles and infertility in males. The activity in seminal plasma was shown to have no effect on complement-dependent sperm immobilizing antibodies in the serum of an infertile woman, implicating an effect on the post-C3 components of the complement pathway. The inhibition of complement-dependent haemolysis and the lack of inhibition of complement-dependent sperm immobilization by the anticomplementary factor are considered in the implications of the role of seminal anticomplementary activity in reproduction.

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