Background and Objective: It is believed that the consumption of an adequate amount of live lactic acid bacteria (as probiotics) may improve the health of their host. Many strains of lactic acid bacteria are generally considered to be safe. However, recent studies have shown that some strains of these bacteria may have adverse effects. Thus, the purpose of this study was to investigate the safety of a new strain of native Iranian Lactobacillus pentosus (IBRC=11143) in male Wistar rats. Materials and Methods: The toxicity of the strain of Lactobacillus pentosus was evaluated in a 28-days sub-chronic toxicity study. Three groups of rats (n=6) received saline or 1 × 10 8 and 1 × 10 9CFU/rat of this stain through gavage once a day for 28 consecutive days. On day 29, under systemic anesthesia, blood, serum and tissue samples of liver and kidney were obtained for analyzing different blood, serum, and tissue factors. Data analyzed with SPSS software. Results: Analysis of tissue sections showed no significant alteration in the liver and kidney. However, a significant rise in ALP level was observed in rat serum. Some blood parameters (Lymphocyte, RBC, and HCT) also showed significant elevations in the groups that received the bacteria. Conclusion: The results indicate the toxic effects of Lactobacillus pentosus (11143) on rats. Therefore, this lactobacillus strain may not be a good choice as probiotic
