Recently, it was frequently demonstrated that fertility of sows after artificially inseminated is lower than after mating. This is associated with a reduced fertilization capacity of overdiluted insemination doses. The aim of this study was to investigate the sperm motility in the semen samples, forming from the ejaculates with high or low protein content, stored in vitro on 17oC for 3 days. Progressive motility was significantly higher (p<0.01) in the ejaculates with high, compared to the ejaculates with low protein content (82% vs. 76%). After 3 days of storage, in the1:4 dilution proportion, the average progressive motility was significantly (p<0.01) decreased in relation to this value in native semen from the boars with high (82% to64%), as well from the boars with low protein content in seminal plasma (76% to48%). However, the average diluted semen progressive motility was significantly greater (p<0.01) in the boars with high (64%), compared to the boars with low protein content in seminal plasma (48%). The number of good diluted semen samples (≥65% progressive motility), was also significantly (p<0.01) greater in the boars with high (41%), compared to the boars with low protein content in seminal plasma (12%). These results show that seminal plasma proteins play an important role in maintaining the sperm progressive motility of diluted semen in vitro stored for 3 days.