Our results of in vivo animal tests showed that the third-gneration AjiPro-L (A3G, Ajinomoto Co. Inc.) has higher bioavailability than the previous generation product (the second-generation AjiPro-L, A2G) mainly due to an increased intestinal digestibility which is attributed to the smaller particle size of A3G compared with A2G. Because the smaller particle size may affect the handling properties such as mixing homogeneity in feeds, durability at TMR mechanical mixing, and stability in TMR contact, however, the following studies were conducted to evaluate those properties of A3G. To examine the mixing homogeneity in feeds, A3G was added into a grain mix at 4.0 wt. % by using an auger mixer. The mix was transported by truck for 100 miles and unloaded into a feed bin. Four sub-samples of the mix (1 kg) were collected from a discharger of the bin. A3G particles in the mix were separated, weighed, and calculated for the inclusion rate. Statistical differences (P < 0.05) were tested by a one way-ANOVA. Mean inclusion rates of A3G in the mix before and after the transportation and subsequent discharge from the bin were 4.3 ± 0.2% and 3.8 ± 0.1%, respectively (mean ± SD, P < 0.05). Assuming the general CV of such feed mixing test is about 5%, those 2 rates fell within an acceptable range of the theoretical inclusion rate ± 2 SD. Next, in situ ruminal protection of A3G after TMR mechanical mixing was evaluated. A3G (1 g) were weighed and placed into a Dacron Bag. The bags were mixed with TMR by the Data Ranger (American Calan, Inc.) and were collected. Then the bags were placed in the rumen for 24 h. Comparing with intact A3G (Control), in situ protection of A3G was not affected by TMR mechanical mixing (P = 0.09). Lastly, a loss of Lys from A3G after contact with TMR was evaluated by measurements of Lys released from A3G into TMR after extraction with water at 0, 6, 12, 18, and 24 h. When A3G was mixed with TMR, about 5% of Lys in A3G was released into TMR within the first 6 h but no further Lys was released during the following 18 h. These results demonstrate that handling properties of A3G are not compromised regardless of reduction in the particle size.