BACKGROUND: Kefir is a fermented milk product containing some anticarcinogenic organic compounds with nutritional benefits, which made it one of the natural dairy beverages extensively consumed. OBJECTIVES: The present study was conducted to assess the effect of two selective probiotics on the values of acidic criteria and probiotic survival in the kefir produced in this study. METHODS: In the first step, the cow milk, preheated at 90oC for 5 min, was inoculated with the commercial starter and divided into two groups. They were complemented with L. acidophilus LA-5 and L. paracasei 431 and incu-bated at 30oC for 6 h. They were then preserved at refrigerated temperature up to 14 days and then sampling was carried out to evaluate the changes of values of organic acids (lactic acid and acetic acid), pH, titratable acidity and survival of probiotic complemented bacteria on the 1st, 7th, and 14th days. RESULTS: The pH values of L. acidophilus LA-5 and L. paracasei 431 were 4.34 and 4.36 at the beginning of the cold storage and reached 4.27 and 4.31 at day 14. The acidity of L. acidophilus LA-5-complemented kefir on the 1st day was 0.80 gr/100 gr higher than L. paracasei 431-complemented kefir which showed 0.72 gr/100 gr. Lactic acid was ranging from 1.57 to 2.40 gr/100 mL or 2.17 to 2.42 gr/100 mL (from the 1st to the 14th day) in the kefirs complemented with L. acidophilus LA-5 and L. paracasei 431, respectively. In the kefirs complemented with L. acidophilus LA-5 and L. paracasei 431, the acetic acid was stable (from 0.11 to 0.13 gr/100 mL) during 14 days but increased in the later (from 0.11 to 0.23 gr/100 mL) .The survival of both bacteria was higher than 7 logs CFU/gr in the kefir. CONCLUSIONS: Adding L. acidophilus LA-5 and L. paracacei 431 can moderate the acidity of the kefir and extend the survival of complementary probiotics at a standard level during two weeks of cold preservation.