The present study investigated the effect of the probiotic strain Enterococcus faecium DSM 7134 (2 x 10(7) cfu per g of feed) (probiotic group), natural humic substances (7g x kg(-1) feed) (HS group), and the combination of both (probiotic + HS group) on the performance, serum biochemistry, and biological activity of leucocytes of broiler chickens. Average body weight of chickens in different phases as well as throughout the observation period under different treatments did not differ significantly. Final body weight and feed conversion ratio (FCR) reached in 41 days of trial were 2542.5; 2568.9; 2610.5, 2516.6 g and 1.93; 1.81; 1.78; 1.88 kg for HS, probiotic, probiotic + HS and control group respectively. The best performance variables including the European efficiency index (EEI) were observed in the probiotic + HS group, where the effect of the probiotic supplementation was improved by HS. There were no significant differences among groups in the serum levels of total protein, glucose, cholesterol, aminotransferase and phosphorus. A significant decrease of total lipids in the probiotic group (P < 0.01) and probiotic + HS group (P < 0.001) groups, triglycerides levels in the HS (P < 0.01), probiotic and probiotic + HS (P < 0.001) groups on day 35, calcium level of the probiotic (P < 0.05) and probiotic + HS (P < 0.01) groups on day 41 and alkaline phosphatase levels in the probiotic group (P < 0.01) on day 14 were observed in comparison to the control group. Significantly higher values of indices of metabolic activity of phagocytes (IMA) after 14, 35 and 41 days of the experiment were observed in the probiotic + HS group (P < 0.01, P < 0.05) when compared with the control group. No adverse health effects were observed in experimental groups The results of the present study indicate that HS and E. faecium DSM 7134 had no significant effect on performance. However an improvement in FCR and EEI was observed which is believed to be due to the significant increase of indices of metabolic activity of phagocytes, caused by HS when combined with. E. faecium DSM 7134.