Antagonism of some amino acids (AA) to the inhibitory effects of other AA (Ile, Phe, and Thr) on the growth rate of mixed ruminal bacteria was investigated. In vitro growth rate of the mixed ruminal bacteria was inhibited when the 3 inhibitory AA (1 mM each) were each added to individual control treatments in which an ammonium salt was included as a sole N source. The inhibitory effect caused by Ile was relieved by addition of Leu or Val (equimolar to Ile), and no significant inhibition was shown when both Leu and Val were added together with Ile. The growth inhibition caused by Phe was also alleviated by supplementing with Trp, and was completely negated by adding Tyr. The inhibitory effect of Thr, on the other hand, was not affected by addition of Lys or Met (which are synthesized using a common pathway with Thr), but was mitigated by supplementation with Glu, Ser, Val, Ala, or Gln. Among the antagonistic AA, Leu, Val, Trp, Tyr, and Glu were indispensable for the maximum growth rate of the ruminal bacteria under the experimental condition of supplementation of amino-N, the removal of which from a mixture of 20 protein AA caused the growth rate to decline. Removals of Ile along with Leu or Val or both, of Phe along with Trp or Tyr, and of Thr along with Glu recovered the promotion of bacterial growth rate. It was concluded that inhibitions of the bacterial growth rate caused by Ile, Phe, or Thr could be antagonized by some other AA (Leu, Val, Tyr, Trp, or Glu), and the role of these latter AA as relievers of the inhibitory effects could explain why they are indispensable for maximum growth rate of ruminal bacteria.