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Persistence of free fluid in the uterine lumen of bitches with endometrial hyperplasia appears to be diagnostic for mating-induced endometritis and is associated with reduced chances of pregnancy. This study investigated the possibility that reduced fertility might be associated with an effect of uterine fluid on sperm. Uterine lavage fluid was collected pre- and post-insemination from normal bitches without ultrasonographically-detectable luminal fluid (n=4), and previously non-pregnant bitches with endometrial hyperplasia and luminal fluid (n=4). Concentrations of polymorphonuclear neutrophils (PMNs) were measured and the effect of the fluid on the attachment of spermatozoa to the uterine epithelium was studied using medium (M) 199 as a control. To elucidate whether any effect was accounted for by the presence of PMNs, attachment was also measured in M199 with PMNs added at the concentration found in lavage fluid. Pre-insemination lavage fluid from both groups contained low concentrations of PMNs which increased post-insemination; the increase was larger for bitches with uterine fluid. Compared with M199 controls, lavage fluid reduced the attachment of spermatozoa; fluid from bitches with endometrial hyperplasia and uterine fluid had a greater effect than normal bitches, and post-insemination fluid had a greater effect than pre-insemination fluid. Spermatozoal attachment was reduced by a similar magnitude for M199 with added PMNs, although post-insemination fluid from bitches with endometrial hyperplasia reduced attachment more than M199 with added PMNs. Poor fertility in bitches with uterine luminal fluid might be partially associated with impaired attachment of spermatozoa to uterine epithelium, mediated principally, but not solely, by PMN influx into the uterine lumen., (Copyright © 2013 Elsevier Ltd. All rights reserved.)