ABSTRACfLactic casein was treated with formalin in an attempt to protect the protein against ruminal degradation. Sheep received diets supplemented with 100 gjday of treated casein. Wool growth and the level of free amino acids in the plasma were compared with those of sheep receiving no supplement or untreated casein. A fourth group of sheep received a supplement of 1 gjday of methionine hydroxy analogue (MHA) mixed with 5 g of casein, the mixture being formalin treated after blending.Substantial wool growth increases from midside areas resulted from both the formalin-treated and the untreated casein supplements. Significant differences occurred in the levels of free citrulline, valine, leucine, histidine, proline, and ,-N-methyl lysine in the plasma. With each of these amino acids the untreated-casein supplemented sheep showed a small increase and the treated-casein group a larger increase when compared with the control group. No change in either wool growth or free amino acids resulted from the MHA supplement.