Binding of monoclonal antibody MHS-5 to western blots of human seminal plasma was employed to follow the fate of a seminal vesicle-specific antigen (SVSA) during semen liquefaction. Ejaculates from four vasectomized donors were collected in a manner to inhibit liquefaction or to allow liquefaction to proceed at room temperature. Aliquots of the liquefying seminal fluid were removed at specific time points and further liquefaction inhibited with sodium dodecyl sulfate (SDS). Western blot analysis using monoclonal antibody MHS-5 demonstrated that the SVSA epitope in all donors was located on multiple bands ranging in mass from 15-92 kDa in unliquefied ejaculates; immunoreactive peptides below 15 kDa were not detected. As early as 5 min post ejaculation, immunoreactive bands below 15 kDa were identified in liquefying samples. During the same time period (5 min), immunoreactive bands of 69-71 and 58 kDa could not be immunologically detected in liquefying samples. A decrease in immunoreactive staining of components higher molecular mass was accompanied by a concomitant increase in immunoreactive staining of intermediate and small molecular mass molecules during the first 2 h of liquefaction. After 8-24 h of liquefaction, two immunoreactive bands of 10.9 and 12.5 kDa predominated. Between 24 and 48 h, each donor's ejaculate demonstrated a common single immunoreactive band of 10.9 kDa. These results indicate that there is a rapid transformation in mass of the SVSA with major 69-71 and 58 kDa bands being converted to forms of lower mass within 5 min of ejaculation.