Young chicks were fed sulfur-amino acid (SAA) deficient soy protein or crystalline amino acid diets to evaluate the relative efficacy of the D- and L-isomers of methionine and 2-hydroxy-4-(methylthiobutyrate)Ca (OH-methionine). With crystalline amino acid diets, D-OH-methionine ([alpha] D25 = +24.3) was decisively superior to L-OH-methionine ([alpha] D25 = -25.3) as a source of methionine activity. The D-isomer of OH-methionine was also much more toxic than the L-isomer as measured by magnitude of growth depression and severity of hemolytic anemia. DL-OH-methionine was intermediate in toxicity between the pure D- and L-isomers, but its efficacy for growth promotion was greater than what would have been predicted from its (equal) content of the two isomers. With methionine, the L-isomer was superior in growth-promoting efficacy to the D-isomer; DL-methionine, however, more nearly paralleled the response to L- than to D-methionine. In all cases where the crystalline amino acid diet was fed, regardless of isomeric form, methionine was vastly superior to OH-methionine as a source of SAA activity. With the soy-protein diet, on the other hand, differences in efficacy between methionine and OH-methionine were minimal, and isomeric form was of lesser consequence.