An experiment was conducted with broiler (Cobb 400) male chicks (n = 480) to determine the effect of betaine (Bet) supplementation (0 and 800 mg/kg) to diets containing five concentrations (15, 18, 20, 22 and 24 g/kg crude protein, CP) of methionine (Met) in a $2{ imes}5$ factorial study for performance, carcass traits, immune responses, and serum parameters. Each diet was fed ad libitum from 1 to 42 d of age to 8 replicates of 6 chicks. Birds were housed in battery brooders placed in an open-sided poultry shed. Body weight gain, feed intake, feed conversion efficiency and slaughter variables were recorded at 21 and 42 d of age. Serum biochemical profile, antibody production against Newcastle disease (ND) and lymphocyte proliferation ratio (LPR) were analysed at 42 d of age. Supplementing Bet to diets containing sub-optimal concentrations of Met (15 g/kg) improved weight gain and breast yield at 21 d of age (p<0.01), and feed conversion efficiency at 42 d of age (p<0.05). Feed efficiency at 21 d of age, body weight gain at 42 d of age, slaughter variables except breast yield at 21 d of age and ND antibody titres were not affected (p>0.05) by the interaction. LPR increased (p<0.05) with Bet supplementation at 20 g Met/kg CP equal to those broilers fed 24 g Met without Bet. Bet supplementation enhanced the concentrations of protein, globulin and cholesterol in serum of broilers fed sub-optimal concentrations of Met. Results suggested that Bet supplementation (800 mg/kg diet) enhanced growth (21 d), feed conversion efficiency (42 d), breast yield and lymphocyte proliferation in broilers fed a diet containing 15 g Met/kg CP.