The aim of this study was to investigate the effect of two levels of whey protein concentrate (WPC, 80.35% protein, 5.86% lactose) added to diets and duration of these treatments on growth and slaughter value, haematological parameters involved in immune processes, oxidative status of liver and breast muscle, and selected indices of gut function of broilers. A total of 560 Ross 308 broilers were randomly allocated into 7 groups with 10 replicates of 8 chickens per replicate. The experiment lasted 42 days and was divided into 3 successive feeding phases. The control group was fed basal diets consisted of maize, wheat and soyabean meal (C group). The other 6 groups received, for 7, 21 and 42 days basal diets with 8 or 32 g · kg^–1 WPC added at the expense of soyabean meal (WL and WH groups, respectively). On day 42 group WH had a higher lymphocyte percentage (P < 0.05) compared to the control. The long-term (42 days) feeding with higher level of WPC significantly decreased the heterophile/lymphocyte ratio (P < 0.05 vs C). The number of erythrocytes on day 42 and the whole blood glucose on day 21 were significantly increased in WH birds than in controls. The concentration of serum total protein on both WPC levels was greater (P < 0.05) on day 21 compared with the C group. The reduction (P < 0.05) in liver thiobarbituric acid reactive substances (TBARS) and a tendency towards lower TBARS level in breast meat were observed in birds WH when fed for 42 days. Only higher dietary WPC amount provided for 42 days reduced the caecal and small intestinal pH values (P < 0.05 vs C). The ileal digestibility of crude protein was higher (P < 0.01) in birds fed with 32 g WPC during the first 21 days of life, compared to the C and WL treatments. The factorial ANOVA showed that on days 7, 21 and 42 the final body weight gain (BGW) and feed conversion ratio in broilers WL and WH were significantly better than in the C ones. Assessment of orthogonal contrasts revealed significantly higher (P < 0.05) final BWG in the WH broilers, compared to the WL dietary treatment; such effect was not observed in earlier feeding stages. Generally, carcass yield on both WPC dietary levels, and breast muscle percentage on WH treatment, were higher (P < 0.01 and P < 0.05, respectively) compare to the C group. In conclusion, WPC added to a conventional diets for growing chickens exerts a growth-promoting action and at a dose of 32 g · kg^–1 may induce desirable changes in bird’s health and the intestinal tract metabolism.