The differential effect of fox and pig bile and its corresponding low molecular weight fraction (LMW) was investigated on the in vitro invasion of MDCK-AA7 epithelial cell monolayers by Trichinella spiralis muscle larvae. Seven invasion experiments were performed and a total of 274 cell monolayers were examined. Fox and pig raw bile at 1:10 and 1:20 dilution and their LMW fractions at 1:10 dilution activated T. spiralis larvae to invade the cell monolayers. In addition, fox raw bile caused significantly larger cell damage than pig raw bile at both dilutions. The area of cell damage was larger at 1:10 than at 1:20 dilution for both fox and pig raw bile (p<0.05). On the other hand, there was no significant difference between the areas of cell damage caused by the LMW fractions of fox and pig bile. It is concluded that differences between host bile actions may account for differences in host susceptibility to T. spiralis.