Abstract: Two-dimensional polyacrylamide gel electrophoresis of seminal plasma samples indicate that two proteins (26 kDa, pI 6.2; 55 kDa, pI 4.5) predominated in higher fertility bulls and two proteins (16 kDa, pI 4.1; 16 kDa, pI 6.7) predominated in lower fertility bulls. A regression model was developed to predict bull fertility using the four fertility-associated protein densities. A plot of actual bull fertility versus that calculated by this model was linear and positively correlated (r=0.89). These findings indicate that bull seminal plasma contains fertility-associated proteins which are predictive of bull fertility. Additionally, the ability of seminal plasma to alter the in vitro fertility of ejaculated bull sperm was examined using a sperm penetration assay for zona-free bovine oocytes. Washed, ejaculated sperm from bulls of below (low) or above average (high) fertility were mixed with seminal plasma from the same bull, or with seminal plasma from a bull of contrasting fertility.