Two experiments (EXP) investigated broiler growth (0-21 d) and tissue glutathione (EXP 2) when fed reduced CP (RCP) diets with different dietary Gly levels and Met:Cys ratios. Ross 708 male off-sex chicks reared in floor pens (12 birds/pen) were used in EXP 1 (1,716 chicks) and 2 (1,344 chicks). EXP 1 included 13 diets: a control (CTL) diet or RCP diets (~3.6 unit CP reduction) formulated to either a high (74:26) or low (60:40) Met:Cys ratio and 1.75, 2.01, 2.27, 2.53, 2.79 or 3.05% total Gly+Ser levels (tGly+Ser). EXP 2 included 6 diets: a CTL diet or RCP diets (~2.6 unit CP reduction) formulated to have either a high (72:28) or low (55:45) Met:Cys ratio and 1.60, 2.08 or 2.56% tGly+Ser. An additional factor in EXP 2 was Eimeria vaccination status [without (UNVAC) or with (VAC)]. Both EXP used one feeding phase. Treatments were rep- licated in 11 (EXP 1) or 16 (EXP 2) pens. Body weight gain (BWG), feed intake (FI), and FCR were evaluated, and in EXP 2, liver, blood and jejunum mucosa from 2 birds/pen of 70 pens were sampled for glutathione analysis. Data from EXP 1 were analyzed by a 2-way ANOVA with linear and quadratic contrasts across Gly levels, while EXP 2 data used a 2- and 3-way ANOVA (P≤0.05). In EXP 1, birds fed the RCP diet had lower (P<0.05) 0-21 d BWG and FI than CTL-fed birds, and increasing tGly+Ser linearly reduced (P<0.001) BWG and FI, but did not affect FCR. In EXP 2, 0-21 d bird BWG, FI and FCR were negatively affected (P<0.001) by VAC, and increasing tGly+Ser improved (P≤0.043) BWG, FI and FCR independently of VAC status. Birds fed RCP diet had inferior performance (P<0.05) than CTL-fed birds. Increasing tGly+Ser lowered FCR of UNVAC, but did not benefit the FCR of VAC birds. The Met:Cys ratio did not interact (P>0.05) with Gly levels or affect bird performance in either EXP. In EXP 2, liver glutathione levels were increased (P<0.05) by VAC, and VAC×Met:Cys interactions were observed for mucosa glutathione.