A new form of methionine, the isopropyl ester of methionine hydroxy analog (HMBi), has been proven to be available for milk protein synthesis in dairy cows. Studies have also shown that HMBi can be degraded by rumen micro-organisms. By two different experimental approaches (emptied and washed rumen; catheterization of the ruminal vein), we have demonstrated that HMBi ability to be rapidly absorbed by the rumen wall can explain its partial protection from microbial degradations. The high efficiency of HMBi to pass through biological membranes seems to be due to the molecule’s isopropyl ester radical.