β-glucans have been shown to stimulate the immune system in several animal species. The aim of this study was to evaluate the immune stimulation capacity of a fully formulated diet with β-1,3-1,6-glucans in cats, by assessing the rabies antibody titer after vaccination. Thirty-five healthy cats were recruited. The cats were placed into two groups and fed a standard diet in accordance with body weight. One group had the β-glucans incorporated into the diet; the other group served as the control group. After two weeks of dietary adjustment; the rabies vaccine (Imrab 3 TF; Merial) was administered on days 0 and 21. Blood samples were taken on days 0, 21, and 42. Titers were determined with the rapid fluorescent foci inhibition test (RFFIT). Titers at days 21 and 42 were compared between the two groups in a linear mixed effects model. This study showed that the animals receiving the non-supplemented feed had higher post-vaccination rabies antibody titers. This indicates that, in contrast to other animal species, the β-glucan supplemented diet did not have the expected positive effect on the rabies antibody titers in cats.