Summary The purpose of this paper is to examine the effect of antibiotic diluent additives on the motility and morphological integrity of diluted fresh boar semen during a six-day storage period. A total of 60 insemination doses, originating from two Landrace boars, were examined and allocated to control (C, n=30, diluted with BTS) and experimental groups (E, n=30, diluted with BTS upon antibiotic addition). The treatment applied exerted positive effects on the preservation of progressive motility, percentage of live sperm and HOS test results (70.24 vs. 66.53%, 71.54 vs. 69.77%, 67.35 vs. 64.17% and 64.10 vs. 54.26%;91.15 vs. 90.02%, 88.38 vs. 85.55%, 81.50 vs. 76.13% and 74.53 vs. 68.72%; and 93.35 vs. 92.40%, 91.04 vs. 88.02 %, 84.67 vs. 78.15% and 77.27 vs. 69.44% HOS+ sperm for the 1st, 3rd, 5th and 6th day of storage, respectively). The results obtained indicate that the treatment applied has a favourable effect on preserving the quality parameters of diluted fresh boar semen during storage, resulting most likely from a reduction of bacterial contamination.