Ninety weanling pigs in Exp. 1 (6.27 +/- 0.73 kg; 21 d of age) and 96 growing pigs in Exp. 2 (21.73 kg +/- 1.29 kg; 56 d of age) were used in two 42-d experiments to evaluate the effect of phenyllactic acid (PLA) on growth performance, apparent total tract digestibility (ATTD) of DM and N, fecal pH value, microbial shedding, and blood profiles. In Exp. 1, the 3 dietary treatments were 1) negative control (NC), 2) positive control (PC), NC + antibiotics, and 3) PLA, NC + 0.5% PLA. In Exp. 2, dietary treatments were 1) control diet (CON), 2) PLA-0.1, CON + 0.1% PLA, 3) PLA-0.2, CON + 0.2% PLA, and 4) PLA-0.3, CON + 0.3% PLA. In Exp. 1, pigs fed the PC and PLA diets had greater ADFI during the overall period (P < 0.05) and tended to have greater ADG and G:F from d 7 to 21 (P < 0.10) than those fed the NC diet. The ATTD of DM was greatest in pigs fed the PLA diet on d 20 and 41, and N digestibility on d 20 was greater in pigs fed the PLA diet (P < 0.05) than those fed the NC diet. The numbers of white blood cell and lymphocyte concentrations on d 42 were increased (P < 0.05) by the inclusion of antibiotics and PLA in the diet. In Exp. 2, G:F tended to increase when PLA was added (quadratic, P < 0.10). The ATTD of DM did not differ among treatments, but there was a tendency (quadratic, P < 0.10) for N digestibility to increase as PLA levels increased. The lymphocyte percentage on d 42 increased linearly as dietary PLA increased (P < 0.05). Additionally, the white blood cell counts on d 42 tended to increase as PLA levels increased (P < 0.10). In both experiments, there was no effect of treatment on the fecal pH or presence of Lactobacillus, but the number of Escherichia coli in feces on d 41 decreased in response to the addition of PLA [P < 0.05 and 0.001 (linear) in Exp. 1 and 2, respectively]. In conclusion, PLA can decrease the number of E. coli, and this novel dietary acid may have potential to stimulate the immune system for both weanling and growing pigs. Thus, it could be a good candidate as an alternative to antibiotics in pig diets.