Effects of feeding supplementary methionine and choline on broiler growth and immunity were examined by supplementing a corn-soybean diet that contained 21% crude protein, 3,255 kcal metabolizable energy/kg diet, .35% methionine, .37% cystine, and .13% choline. Methionine (.063, .125, .25%) and choline (.125, .25%) were dietary variables. Sulfate (.055%) was added either alone or along with methionine (.125 or .25%) and choline (.125%). In one study, the .25% methionine diet was supplemented with .121% betaine. Sodium and chloride levels were constant in all the diets. Feed and distilled water were supplied ad libitum. Total antibodies, immunoglobulin (Ig) G (2-mercaptoethanol-resistant antibodies) and IgM (2-mercaptoethanol-sensitive antibodies) were determined in 3-wk-old chicks inoculated intraperitoneally with sheep red blood cells. The thymus-derived (T)-cell-dependent in vivo mitogen response to phytohemagglutinin-P (PHA-P) was assessed via wing web swelling. The methionine requirement for growth (0 to 3 wk of age) was approximately .413% of the diet (.35% in the basal diet plus .063% added). Supplementation of the basal diet with .125% choline stimulated growth to the same extent as did the extra .063% of methionine. Addition of .055% sulfate with .125% choline did not improve the ability of the latter to spare methionine. Supplemental methionine resulted in significant (P less than .05) dose-related increases in total antibody, IgG, and response to the mitogen PHA-P, but not in IgM. There were no effects of choline on the immune variables studied. These results suggest that methionine is required for select components of the antibody response, which effect might be related to T-cell help.