Human ovarian preovulatory follicular fluids (FF) from 10 women were analysed for their complement contents. Functionally active complement was detected in all the fluids studied in amounts similar to those present in normal human serum. Pooled FF was challenged by seminal plasma in order to determine whether seminal plasma could activate FF complement, the pattern of such an activation and the possible consequences on the reproductive function. FF complement activation occurred during the incubation with seminal plasma with features including alternative pathway activation, factor B and C3 conversion and reduction in total haemolytic complement, as well as an inhibition by seminal plasma of the FF complement response to a new activating challenge. Possible consequences for fertilization, implantation of a fertilized ovum and local defence mechanisms against viruses and bacteria are discussed.