The aim of the study was to test whether the use of probiotic bacteria Bacillus subtilis or Enterococcus faecium or a phytobiotic containing cinnamon oil can improve the metabolic parameters, immune status, gut microbiota and histology, and growth performance of broiler chickens. The experiment was carried out on 560 one-day-old male Ross 308 broiler chickens raised until the age of 42 days. The broiler chickens were assigned to 4 experimental groups of 140 birds each (7 replications of 20 individuals each). The control group (Control) did not receive additives. A probiotic preparation containing live bacterial cultures of Enterococcus faecium (EF, in the amount of 0.25 g/l) or Bacillus subtilis (BS, 0.25 g/l) or a phytobiotic preparation containing cinnamon oil (OC, 0.25 ml/l) was administered to the broiler chickens with their drinking water throughout the rearing period. The most important results indicate that the use of BS and OC resulted in: a significant (P≤0.05) increase in the level of ferric reducing ability of plasma (FRAP), high-density cholesterol (HDL) and glutathione (GSH + GSSH) and a significant (P≤0.05) decrease in the level of malondialdehyde (MDA), lipid hydroperoxides (LOOH), total cholesterol (TC), triacylglycerols (TAG), nonesterified fatty acids (NEFA) and interleukin 6 (IL-6), a ratio of heterophils : leukocytes (H:L) and alkaline phosphatase (ALP) activity, alanine aminotransferase (ALT), acidic phosphatase (AC) and creatinine kinase (CK), relative to the C group. In the blood of broiler chickens from the OC treatment, aspartate aminotransferase (AST), lactate dehydrogenase activity and 3-hydroxy-butyrate dehydrogenase (HBDH) significantly (P≤0.05) decreased in relation to the C group, and in broiler chickens from EF and BS treatments there was an increase (P≤0.05) in haemoglobin (Hb) content. Compared with group C, in the broiler chickens’ nutritional content from EF, BS and OC treatments, the total number of coliforms and number of fungi significantly (P≤0.05) dropped and the number of aerobic bacteria increased (P≤0.05) in the length of the villus and the depth of the crypt. It has been found that Bacillus subtilis, Enterococcus faecium and phytobiotic containing cinnamon oil can improve the microbiological and histological appearance of broiler chicken intestine. The addition of probiotic bacteria Bacillus subtilis or phytobiotic containing cinnamon oil to drinking water is more preferable than Enterococcus faecium regarding stimulation of the immune system, blood redox status parameters, parameters of metabolic changes and the gut microbiome and morphometry.