The aim of the study was to select a dosage and time of administration of a probiotic preparation containing live cultures of Bacillus subtilis and enriched with choline to obtain the most beneficial effect on the antioxidant and biochemical status of the blood of chickens and to improve their growth performance. A total of 980 one-day-old Ross 308 chickens (7 replications of 20 individuals each) reared until their 42nd day of life were used in the experiment. The chickens were divided into seven groups of 140 each. The control group did not receive any additives. The T1 groups received a probiotic in the amount of 0.05 g/L (T1-0.05); 0.1 g/l (T1-0.1) or 0.25 g/l (T1-0.25) throughout the rearing period, while the T2 groups received the same doses of the probiotic, but only during days 1–7, 15–21 and 29–35 of rearing. Administration of a preparation containing Bacillus subtilis bacteria was shown to increase the level of ferric reducing ability of plasma (FRAP), vitamin C, and uric acid (UA), while reducing the level of peroxides (LOOH), malondialdehyde (MDA), nonesterified fatty acids (NEFA), the share of and low-density fractions of cholesterol (LDL), and activity of alanine aminotransferase (ALT), asparagine aminotransferase (AST), γ-glutamyltransferase (GGT) and creatinine kinase (CK). An increase in the high-density fractions of cholesterol (HDL) fraction of cholesterol and a decrease in lactate dehydrogenase (LDH) and alkaline phosphatase (ALP) were noted as well. The results of the study indicate that 0.25 g/l of the probiotic, administered continuously (T1), clearly has the most beneficial effect in terms of enhancing antioxidant potential and reducing the level of stress indicators, without disturbing overall metabolism in the body. During the 42 days of rearing each chicken received 33.3 CFUx1011 Bacillus subtilis from the probiotic preparation. The body weight gain of chickens from T1-0.1, T1-0.2 and T2-0.25 groups was higher (p≤0.027) and more favorable comparing to G-C group.