The study was conducted to investigate the effects of mannanoligosaccharide (MOS) and probiotic supplementation on haematological and immunological parameters in turkeys. A total of 72, 15-day-old white hybrid converter turkey poults were used in this study. Poults were assigned into three groups, each group consisted of six poults and the trial was repeated four times. The control group was fed a basal diet without supplemented probiotic and MOS, and treatment groups were fed either 1 g/kg MOS or probiotic added diets. At the end of the 15-week treatment period, samples of blood were collected to determine immunological and haematological values. The comparison among the groups showed that both the probiotic and MOS supplementation resulted in significant increases (P < 0.05) in the serum IgG and IgM levels, and significant decreases (P < 0.05) in the peripheral blood T lymphocyte percentage compared with those of the control group. Mean serum IgG level (7.06 g/l) was significantly higher (P < 0.05) in poults supplemented with probiotic compared with MOS (6.76 g/l). It was observed that the probiotic supplementation caused statistically significant increases (P < 0.05) in the erythrocyte count, haemoglobin concentration and haematocrit values, but MOS supplementation did not have a significant effect (P > 0.05) on these parameters. Total leucocyte and differential leucocyte counts were not affected by dietary MOS and probiotic supplementation. These results show that MOS or probiotic may elevate IgG and IgM levels in turkey. The MOS and probiotic that enhance immunoglobulin levels will have a more positive effect on growth performance, production and turkeys' ability to resist disease.