A liquefraction factor from normal human seminal plasma was purified by cation and anion exchange chromatography. It is nondialyzable, heat-labile, and has many properties in common with the seminal protease "chymotrypsin-like enzyme." Its lytic effect is most distinctly observed in abnormal semen that is semisolid or that is solid and fails to liquefy for several hours. Seminal plasmas from certain of these samples have and adverse effect on normal sperm motility. Clinically, the purified seminal liquefaction factor is useful in solubilizing abnormal samples for sperm analysis and homologous artificial insemination.